linkmap: Reduce radius on culled chunk drawing to get a flat section
linkmap: Cairo will implicitly join strokes, so clean up these
gutterrendererchunk: Move action map into class
gutterrendererchunk: Refactor some ugliness and create new consts module
settings: Add bind_settings helper for attribute-specified properties

The idea here is to have settings bindings specified next to the
GObject attributes that they're often associated with. This commit
adds the helper, and moves all of the major uses of gsettings binding
to it.
build_helpers: Fix accidental tabs
meldwindow: 'Fix' property warning