diffmap: Update Diffmap to match its scrollbar's width
meldapp: Quit correctly
diffmap: Cache the main diffmap drawing to avoid scrolling redraw
filediff: Remove old invalidate/process code for Linkmaps

This used to exist to give nicer scrolling behaviour to synchronise
Linkmap updates with the scrolling of the ScrolledWindow. Now, it just
imposes a massive performance penalty and doesn't seem to really
improve matters.
recent: Remove RecentFiles argument so we can move it from MeldApp
filediff: Refactor next_diff
filediff: Simplify our key press/release event handling
Move recent files management off the app
ui: Remove old glade-integration stuff and fix imports