Use "Set value" as title for QInputDialog of some widgets.

[Sawuare]

These widgets are the Fader, Knob and LcdSpinBox.
The QInputDialog shows when a widget from the above is double-clicked.
I propose this change because the current title windowTitle() usually doesn't have a specified value and defaults to lmms:

With this change, it will always default to Set value:

[tresf]

This looks good. In regards to the way we're handling this, I feel we may be able to consolidate the redundant strings. @liushuyu any thoughts?

[Wallacoloo]

1. Given how both LcdSpinBox and Fader already duplicate the tr( "Please enter a new value between %1 and %2:" ) string when calling QInputDialog, I think it's not that much worse duplicating the other strings for now.

2. The easiest way to consolidate these calls is to create a new method getInput on the ModelView class, which creates and populates a QInputDialog. I'll try to look into it this week.

[Wallacoloo]

I went ahead and merged this as per my point 1 above. I still plan to consolidate these - but when I took a first pass at it today it turned out there's some cleanup around ModelViews/Models required to make the consolidation work well, so it's going to have to happen incrementally.