Minor comments updates
Make symmetry state properties of the root stack

Symmetry state (whether symmetrical painting is active, and the axis
position) are global properties of the document structure, and must be
reflected into layers when things change to keep the brush engine
informed.  The natural place for them is the root of the layer stack.

Deprecate the old methods for setting the symmetry axis.
Algorithm: intersection of two line segments
Active mode: change observer list to event

Minor internal API change.  Transient objects may be interested in the
active mode changing so it's best to observe without creating circular
reference chains.
Cursor API changes

Move custom cursor generation and caching to gui.cursor.

Constants for cursor names.

Cursors for adding and removing items. Grab these from the OpenSUSE art
repo before their Expat ("MIT") -> CC-SA license change.
Cursors: directional move cursor functions

Abstract out the functions used for obtaining the special move cursors
used by modes that handle constrained moving of objects on the canvas.
Stylistic tweaks + typo fix
Drag modes: pass through TDW when dragging stops

For a more consistent UI, and because it's often useful to reset cursors or
redraw the canvas at the end of a drag.
Central style vars for consistent appearance

Editing modes are getting more complex, so we need to fight inconsistency
for their knobs and handles.

- Add a central module for graphical style constants
- Begin using them in various places
Some additional drawing routines
Overlays: port to GTK3, use central style vars
Frame tool: style revamp

Thinner edges and standard colours for the resize controls.

Folks were complaining that the orange was ugly.
Undo stack: remove deprecated API

Nuke some calls that don't *seem* to be needed any more.
Update usage of the old symmetry API

The symmetry-active flag is now exposed as a (mostly) independent property.
The toggle action can just poke that, still making sure there's a sensible,
visible axis position if it hasn't been placed yet.
Make symmetry feedback non-optional

It has permanent aspects for all painting modes, and is invisible when
symmetrical painting is inactive. It doesn't make sense to allow the user
to turn it off.
Rename the symmetry toggle action
Floating action button rendering function
Add a real symmetry axis setting tool

Update the toolbar "butterfly" button with a tool for editing the symmetry
axis position. Initial feature list:

* Sensible initial location
* Trash button for getting rid of the symmetry axis
* Click to set location
* Pretty? Styles should be more consistent with other things

Fixes #9.