The extra transparency was conflicting with the positioning of
the arrows in the layout

MixerChannelView is now a combination of the
MixerLine with the previous MixerChannelView

- Move MixerChannelView into src/gui

Change MixerView.h to use MixerChannelView rather than MixerLine
Also do some cleanup, such as removing an unused forward declaration
of QButtonGroup

+ Set height of sizeHint() using MIXER_CHANNEL_HEIGHT (287)

- Include MixerChannelView

Before, MixerChannelView was not inherited by QWidget,
meaning it could not have a parent and had to be deleted
when necessary. Since the MixerView owns the
new MixerChannelView, this is no longer necessary.

- Include MixerChannelView in MixerView

…codebase

- Set the Qt::WA_StyledBackground attribute on

- Change nullptr to this for certain widgets
   - Some custom widgets may expect there to be a parent
- Add spacing in channel layout
- Increase size of mixer channel

- Rename states in SendReceiveState

- Make maxTextHeight constexpr in elideName
- Remove background changing on mouse press
(is now handled in paintEvent)

Not sure if that connection with the mute model was needed, but removing
it did not seem to introduce any issues.

+ Make m_renameLineEdit transparent
+ Retain size when LCD is hidden
+ Remove stretch after renameLineEdit in layout

+ Transpose m_renameLineEditView rectangle (with 5px offset)

+ Leave auto fill background to false in MixerChannelView
+ Only set width for EffectRackView

Finxing the build mostly includes applying several of the changes that have been made to `MixerLine` in the meantime to `MixerChannelView`. The relevant commits are:
* Classier enums: f102777
* Keep master bus color on FX Mixer when switching project: 609c008
* Use std::optional for colour interfaces and storage: dc8c49a

Commit aa050ae also made changes to `MixerLine`. These have not been applied as they do not seem to be relevant to `MixerChannelView`.

`InstrumentTrackView` and `SampleTrackView` had references to `MixerLineLcdSpinBox`. These have been adjusted to `MixerChannelLcdSpinBox` in the header files and the implementation files.

Conflicts:
* include/MixerChannelLcdSpinBox.h
* include/MixerLine.h
* include/SampleTrackWindow.h
* src/gui/MixerLine.cpp
* src/gui/MixerView.cpp
* src/gui/SendButtonIndicator.cpp

Move the solo and mute buttons closer to each other in the mixer channels.

Technically this is accomplished by putting them into their own layout with minimal margins and spacing.

Mostly whitespace and formatting changes: remove tabs, remove spaces in parameter lists, remove underscores from parameter names.

Some lines have been shortened by introducing intermediate variables, e.g. in `MixerChannelView`.

`MixerView` has many changes but only related to whitespace. Spaces have been introduced for if and for statements. Whitespace at round braces has been removed everywhere in the implementation file even if a line was not touched by the intial changes.

Remove duplicate forward declaration of `MixerChannelView`.

Make the parent `QWidget` the first parameter as it is a Qt convention. The default parameter had to be removed due to this.

Move the style of the `QGraphicsView` for the rename line edit from the code into the style sheets of the default and classic theme.

Fix spaces between types and references/pointers, e.g. use `const QBrush& c` instead of `const QBrush & c`.
Remove underscores from parameter names.
Remove spaces near parentheses.
Replace tabs with spaces.
Introduce intermediate variable to resolve "hanging" + operator.
Replace the connection for the periodic fader updates with one that uses function pointers instead of `SIGNAL` and `SLOT`.