[Aggregator] Non-zero notifications, fixes #1246.
[Aggregator] Moved shortcuts manager to the core for #1243.
[Aggregator] Somewhat fixed style.
[Aggregator] Default shortcuts for item actions for #1243.

Set default shortcuts for "mark as (un)read", "mark as important" and
"delete" item actions.
[Aggregator] Register item actions as shortcuttable for #1243.
[Aggregator] Prev/next item actions, closes #1243.
Merge branch 'master' of github.com:0xd34df00d/leechcraft