Store the CrawlerWidget geometry with the session.
Implements Firefox style tab shortcuts.

(Shift)-Ctrl-Tab: Switches to next/previous
Alt-Number: Switch to

Fixes #62
Add website to About box.

Fixes #58
Improvement on tabs shortcuts.
Add a background task handler to the MainWindow.
Initial implementation of the version checker.
Initial implementation of version comparison.
Introduces a dialogue box to notify of a new version.
Implement scheduling of the version checking.
Make the version checker a build option.
Typo fix in makefile.