Add check of "at least one device enabled" to handle corner case

[WesleyBlancoYuan]

Hi, I love this app but here's one inconvenience that I observed(Ubuntu 18, Gnome, X11):

• If I turn on my laptop, and plug in my mouse, touchpad is disabled, and when unplugging mouse, touchpad enabled again(good)
• If then I disable touchpad with my mouse plugged in, and then I unplug mouse, touchpad still disabled(bad). In this case, no mouse device can be used, until I plug in my mouse. This is somehow inconvenient, because if my mouse is broken and don't have another at hand, I cannot turn on the touchpad with anything; I have to reboot.

Notice under these 2 cases, exist some inconsistency of behaviour. Not bad, just inconsistent.

Could something like a "at least one mouse device enabled" to be add, so that the user always has something to use? Maybe this is not this app's responsibility but still nice to have.