Widget highlighting and shortcut system implementation

[szeli1]

This PR contains multiple changes and overall it is focused on improving lmms functionality communication with the user and on immediate feedback to the user.

These changes are the following:

• The StringPairDrag system and the StringPair system now uses a unified enum as key instead of custom strings. Before this, key names weren't documented and they were scattered across multiple files.
• A new class was added that implements handling shortcuts and highlighting widgets:
  • This class was designed to encourage developers to implement shortcuts and pasting data into the widget.
  • Shortcuts:
    • Shortcuts support all QT keys and modifiers and they can differentiate between how many times a shortcut was pressed. Each shortcut has a description.
    • Shortcuts aim to be easy to work with.
    • If the user moves the mouse over the widget, then the available shortcuts will automatically be displayed.
    • In the future settings could be added that could allow the user to change the shortcuts.
  • Highlighting:
    • The idea behind highlighting is to show users which widget can accept which StringPair key while dragging or copying. If the user starts to drag a clip, the other clips will be highlighted to show that they can accept the datatype associated with the drag event.
    • Highlighting will be active for 10 seconds after the user starts to ctrl + drag a widget or when a widget is copied.
    • Highlighting will stop after a successful paste, but the user can keep pasting into other widgets.
• The new class is implemented inside Knobs (FloatModelEditorBase) and all clips. I plan to implement it in other widgets in the future (if this gets merged).

Problems:

• For shortcuts to work the new InteractiveModelView widgets redirect PianoView's PianoView::keyPressEvent to themself. This solution works but it isn't ideal.
• ClipView::getClipStringPairType and TrackView::getTrackStringPairType does not implement Special types of tracks (video, event)
• I removed ClipView::dropEvent copying code because I found that this condition is never true:

// Defer to rubberband paste if we're in that mode
if (m_trackView->trackContainerView()->allowRubberband() == true)

Picture showing highlighting (for knobs and clips separately):

[messmerd]

I haven't looked over all the code yet, but here are some of my initial thoughts

[messmerd]

The name could probably be shortened to just DataType or something.

[szeli1]

Which one is better: KeyType or PairType or PairDataType or maybe CarriedDataType? I think DataType doesn't say anything about the StringPairDrag system and it could be confusing because there is also a Default mime type, but I guess this can be fixed with a comment.

I would like to get a finalized name because this change would effect 30 files.

[messmerd]

It would be nice if the size_t shortcutLocation parameter could be const ModelShortcut& instead.

[szeli1]

I would like to keep the size_t because I think it is easier to use in this case (with switch) and it could be more optimized (checking if size_t == size_t instead of ModelShortcut == shortcutArray[index]).