Compile with Qt 5
Removed Palette base class

Shared palettes are not planned.
Ripped out old code in preparation of rewrite

The program compiles, but doesn't work right now.
Source code reorganization
Rewrite (WIP): simple drawing

Simple drawing (brushes) now work in local mode.
Rewrite (WIP): drawing progress.

Image loading now goes through the network
Rewrite (WIP): layers work again
Moved brushpreview to widgets/ directory
Better canvas caching
Image loading fixes
Fixed layer stack flattening
Line and Rectangle tools work again
Color picker works again
Annotation editing works again

As a bonus, annotations now support rich text.
Added a reference counting pointer for messages
Properly mark layers as dirty on fill()
Fix crash on layer stack painting
Dualcolorbutton visual improvements
Restored brush outline drawing
various fixes/tweaks