user-accounts: Rename UmCropArea to CcCropArea

Cheese has an internal copy of UmCropArea, and exports the corresponding
um_crop_area_get_type() as part of libcheese-gtk. This leads to a crash
when taking a photo in the avatar chooser, as the control center copy of
the get_type() function is used. Renaming the function in Cheese would
be an ABI break, so at this stage it is best to do the rename in the
control center instead.

https://bugzilla.gnome.org/show_bug.cgi?id=697039