batterymenu: store devices in device_table using object_path strings instead of serial numbers.
Do not create the tray icon if the system doesn't need it.

Likewise, destroy it after the last battery has been removed.
Note that in desktop PCs with e.g. wireless mice/keyboards but not internal battery/ups no icon is shown in the tray.
This needs to be fixed :)
Less verbosity
Merge branch 'master' into debian
0.22-1