editor: rename TrackView to TrackArea
editor: factor out interpolation-pen selection
editor: simplify painting
editor: use QRect for selection
editor: move document-implementation into source file
editor: store everything needed in undo-commands
editor: use Qt's undo framework

Instead of hand-rolling our own systen, let's use Qt's built-in
one to reduce the amount of code.
editor: selection-helper
editor: hide undo-stack
editor: use getTrackFromX instead of repeating logic
editor: do not autoscroll with zero-width trackview

This fixes a glitch where the edit-track is invisible when loading
documents from the command line.
player: do not emulate inline as static

All our inline functions are already declared as static inline,
and multiple static declarations gives compiler errors. So let's
just define inline empty.
player: build non-client variant from makefile
editor: do not track width/height of TrackArea

We can ask the viewport about it instead.
more getTrackFromX
editor: do not track windowTracks
editor: per-pixel horizontal scrolling

This feels a bit nicer.