Add great keybindings

I'm thinking of

• <escape> to go back to the course list;
• <up>, <down> next/previous file, as <tab> currently does;
• <c-pageUp>, <c-pageDown> to switch to the previous and following course 593ac168
• <c-begin>, <c-end> to switch to the first and last course bed95813
• <a-up>, <a-down> next/previous week;
• <a-left> switch back to the course switcher;
• <a-right> to cancel a <a-left>: return to the course window (with the current element who may or may not be selected);
• <c-r> to reload;
• <c-p> might lauch a custom batch print where you select several files and press send. Pro: it's convinient. Cons: it's non-standard.
• <c-c> might allow to copy a file, but I'd say the extra complexity isn't worth it
• typing directly should allow searching, as is a standard on gnome, but it disallows hotkey, which sucks. In all cases, the <c-f> keybinding will be mapped.

I'm wary of remapping tab, so I wouldn't change the default behaviour.

Thoughts on this?