Expanded undo system

[Zweibach]

Undo single file deletion.
Undo remove from view
Undo deselection (having selected a bunch of files or tags, accidentally clicking something else and having them all deselected so you have to redo the selection again)

Dev: Add more commands to the undo system

[bbappserver]

#348

[bbappserver]

Ideally give each tab an QUndoStack, and have the undo views and keyboard listeners affect the "selected undo stack" where this undo stack is the stack of the tab currently in the foreground.
https://doc.qt.io/qt-5/qundostack.html#undo-and-redo-actions