More pleasant and fluid font size

Making the view switcher size depend on the general font size is easier
to change (it adapts automagically). And not using bold looks better.
Fix compile with GTK 3.10. Ugly hack to be removed
GTK3: fix drawing areas && make them really square
Zonesystem IOP: move DT logo drawing into "size-allocate" callback
Zonesystem IOP: "size-allocate" callback: protect from memory leaking

In reality, we do not allow resizing of side-panels,
so this probably would never happen.
cache: make jpg removal thread-safe and more efficient

doesn't first write invalid thumb and then delete it, but just delete it.
Merge remote-tracking branch 'origin/gtk3' into cache2