Add an new command 'rescan' that may be sent to a running
instance of the GUI to force it rescan the config folders.

Use case: with an instance of the is GUI running, one can
manually copy a config file to the config folder and start
it using "openvpn-gui --command rescan" followed by
"openvpn-gui --command connect foo"

v2: The calls to rebuild config file list and recreate
menus is refactored into a function.

Signed-off-by: Selva Nair <selva.nair@gmail.com>