Read and write the settings only from the preferences dialog.
Remove all the copies of some settings which were stored in nearly every
dialog class individually.
Simplify the settings dialog code by removing all those not really
needed slots.
Rename the files and classes for the dialogs to share all the same
naming pattern. This should make navigation in the code a bit easier.
Do not include dialogs not rewritten yet; they'll be edited as they are
redesigned.
