Press enter to start stopwatch

[Thenujan-0]

Add the ability to start stopwatch by pressing enter on the stopwatch tab

[ryonakano]

Enter key is reserved to activate currently selected widget in Gtk, so we should use another hotkey for this

[ryonakano]

Also, it would be even useful if we can use that hotkey not only starting the stopwatch but also stopping it, which means toggling the stopwatch