Add notifications for changes in the wiki

Notifications for:

• new documents
• changed documents
• new items in the Recent Changes
• new items in the Watchlist
• new documents for watched tags

Each type of events can be configured.

Notification types:

• flash/change the statusbar icon of the extension
• textual notification in the status bar, next to the icon
• popup for X seconds (also configurable, 0 means until manually closed)