Fixed an incompatibility with newer version of GTK.

Menu items were not activated, with an error message such as:
show_prefs() takes at least 3 arguments (2 given)
Merge branch 'master' of git://github.com/mitya57/command-runner-applet

Port to GSettings, GAction and PanelApplet 5.0.
Add a gconf to gsettings data migration file and mention it in README.
Added a couple of example commands to README.
Bump version to 0.4.
Port to Python 3.

* Use `except Foo as bar` instead of `except Foo, bar` (this is also
  compatible with Python 2).
* Decode command output as UTF-8.