Float properties can not be read from keyfiles. This was causing changes
to this property in settings.ini to be ignored. Fix this by changing it
to a double.

Technically this change could be considered an API break, but there are
no users of this property as float yet and it is early enough to expect
there to never be any.

Also document this change in the 3 to 4 migration guide since in gtk 3
this setting will be kept a float.

Fixes #3441