filediff: Fix animation colour handling
tree: Fix colour handling for rows in folder and VC comparisons

Gdk.Color and Gdk.RGBA values in TreeStores seem to get corrupted
whenever they're inserted, so we get to fall back to using strings
for now.
ui.notebooklabel: Fix tab close button sizing

CSS taken from gedit's implementation of a similar tab button style.
meldwindow: Style toolbar appropriately