flatpak: Suffix app ID with `.Devel`

This commit changes the app ID from org.gnome.SettingsDevel to org.gnome.Settings.Devel, as recommended by the GNOME Developer Documentation - https://developer.gnome.org/documentation/tutorials/application-id.html#application-id-for-flatpak-development.

Additionally, some changes have been made in the buildsystem, to properly substitute the app ID depending on the profile in use.