Split away/idle prefs into a separate widget

I didn't do any re-designing, but did change `GtkFrame` to `HdyPreferencesGroup`, as that allowed using a `HdyPreferencesPage` like with the credentials page.

Testing Done:
Opened Preferences and didn't see any errors. Also, changed all settings and saw that debug log showed they were changed and prefs were re-saving.

Reviewed at https://reviews.imfreedom.org/r/1435/