start backend with non-root privileges, restart backend when root is needed

there are two cases which need root:
- executing a transaction (updates, undo/redo history, ...)
- permanently enable/disable a repository
add status icon with popup menu to search for updates
status icon: set tooltip to "Yum Extender" on startup instead of "Yum Extender: No Updates"
add --start-hidden command line & configuration parameter: start with hidden main window
add auto-check for updates with configurable interval
status icon: use bold font & a smaller font size
add auto update options to preferences dialog
options dialog: use gtk.SpinButton for integer values instead of gtk.Entry
Merge pull request #6 from bkueng/update_checking

add status icon & auto-check for updates