Delete button for saveloadwindow, make this window less error prone (and some more)

[Max5377]

Adds delete button for saveloadwindow to allow user to delete any save in user\\Documents\\Pioneer\\savefiles\\ folder.
For this, adds new static method Game::DeleteSave(const std::string& filename) and two new methods in FileSystemFS class:

1. FileSystemFS::RemoveFile(const std::string& relativePath), to remove file in the FileSystemFS::m_root + relativePath. Deletion is restricted to only delete regular files;
2. FileSystemFS::IsChildOfRoot(const std::string& path) to check if given path is in the FileSystemFS::m_root.
   Makes saveloadwindow less error prone, especially when trying to save/load. Will show MessageBox with a text about why save/load failed.
   Before saving will check filename with new FileSystem::IsValidFilename(const std::string& filename) if filename has forbidden OS characters.
   Other changes include optimization for Game::CanLoadGame (just tries to open file to check it's existence) and Game::SaveGame (tries to open file first, if failed, immediatlly throw exception), new MessageBox type and little refactoring for LuaGame.cpp.
   Demonstration (Linux VM, similar functionality in Windows):

Delete.Button.mp4

Update:

1. saveloadwindow now shows MessageBox with confirmation if user really wants to delete save. Next it will show another MessageBox, that tells if save deleted successfully or not;
2. Some optimization changes for FileSystemFS::IsChildOfRoot(const std::string& path).

[bszlrd]

Nice!
It might be useful to have some kind of confirmation for deleting to avoid any accidental deletes I think.

[Max5377]

@nozmajner It has, just not used for now. I want to show MessageBox with the error if something goes wrong, but if I show MessageBox with confirmation first, MessageBox with the error will be just not shown. Added comment in saveloadwindow.lua about it:

-- XXX Not used for now, because lui.COULD_NOT_DELETE_SAVE has no time to show itself
-- do to creation of it on top of lui.DELETE_SAVE_CONFIRMATION which deletes itself
-- by the time of showing
local function showDeleteConfirmation()

Edit: correct me if I wrong.
Edit2: workaround for that was actually easy. Going to do push later.

[Max5377]

Hope this is last, missed some copypaste error since last time. Sorry for a lot of pushes.

[Web-eWorks]

Rather than displaying another popup that the user has to click through telling them that deleting a save succeeded, I'd recommend simply drawing the status message right-aligned on the same line as the Save: text. That would put it directly above the Save and Delete buttons where the user expects to see it.

Generally, you want to only interrupt the user's flow to ask them if they're doing a potentially-destructive operation by accident - informational notices should not interrupt the player. If the save or delete operation failed, that can be a valid case for displaying a popup notification, but deleting a savefile shouldn't take three clicks.

[Max5377]

@Web-eWorks

DeleteSuccessfulText.mp4

[Web-eWorks]

Looks good! I'd recommend using the next smaller font size breakpoint (with ui.withFont(<font>, function() ... end)) and adding some spacing between the text and the Load buttons, but otherwise I think it will work.

[Max5377]

@Web-eWorks
Changed font size to small and added padding between Load button and Save deleted successfully.
Also added new pigui lib ui-timer to create timers that work from internal ImGui timer. For example, can be used to show something for a limited amount of time (like with Save deleted successfully).
Don't really know if it's good for a solution, but will change it if needed.

[Web-eWorks]

Finally reviewing this now that I have the time available, thank you for the patience.

Code looks good, the new methods in FileSystemPosix/FileSystemWin32 could probably receive another pass at some point for optimization and cleanup (for example, the forbidden characters list on unix is definitely missing a few characters), but overall I think it's fine.

Tested on Linux, if anyone wants to give this a test on Windows before I merge, let me know in the next 24 hours or so.