topbar: fix styling in some themes

Rework the way the menu button repacks, raiding git for the old "window
sandwich" approach and modernizing its code. No custom class assignment
during repacks now because things look too weird if we do: borders on
toolbar buttons and other ugliness.

Tested in: Numix, Orion, Albatross, Adwaita, Clearlooks, Mediterranean
Night, Greybird, Zukitwo. All as of 2013-10-17 under GTK 3.8.

Add test code for the topbar. No API changes.
paletteview: fix exception when opening properties

Fixes https://gna.org/bugs/?21203
fill: fix mem leaks, add debugging checks

The fill routine was being way too lax about reference counts.
Merge Dopey with MyPaint HEAD