Ensure IBus daemon autostarts if (and only if) input methods are active

[jeremypw]

Fixes #https://github.com/elementary/wingpanel-indicator-keyboard/issues/105 fixes #379

• When the IBus daemon is started manually a suitable desktop file is added to the user's autostart directory
• Whether autostart is enabled depends on whether there are any input method engines active.

At the moment IBus is not autostarted, even if input method engines are added, unless the user activates the daemon using the "No Daemon" view.

At the moment, Wingpanel has a switch to turn the IBus daemon on and off temporarily but this setting is not remembered between sessions - IBus is still autostarted. It should be considered whether turning this switch off should disable the autostart file - which would require another PR for wingpanel.

[jeremypw]

@ryonakano Thank for the review. I copied a lot of the code from the switchboard startup applications plug and most of the code style issues pre-existed there. I'll clean it up.

[jeremypw]

@ryonakano Thanks for the helpful suggestions - I have updated PR accordingly.