Various small changes

1) No icon in panel.
2) Handle on_applet_removed_/on_applet_added_.
3) Don't update label if nothing has changed.
4) No try-catch in _init.
Add CPU-usage string to label.
Hard-code a width for the applet, so we don't shrink and expand dynamically.
Add accumulated CPU usage statistics.
Add accumulated total CPU usage statistics.
Merge pull request #1 from autarkper/pull

Add CPU monitoring, plus some other changes