gtkrc, filediff: Update Meld's colour scheme slightly

While this isn't final, our colours aren't great. One of the biggest
issues is that we set a foreground colour in inline highlighting,
which is guaranteed to clash with any use of syntax highlighting.
diffmap: 'Fix' Diffmap sizing for some weird DirDiff race condition

This isn't a real fix; for some reason even when a resize is queued
with the correct width, the draw event comes through while there is
still an old requisition on the widget. This isn't reproducible if
the allocation is >1, so it looks like it might be some weird
optimisation edge case?
diffmap: Force a repaint of the map background when we resize
Migrate to using GtkStyleContext everywhere for colour lookup
bin/meld: Update to use GtkCssProvider instead of gtkrc