Rename widgets to reduce confusion
Revert "We don't have to specify a width for the menu"

Actually, we do for any position other than composite menu.
Prefs: Fix replacing of underscores

There might be more than one underscore in a preference.
Rework handling of settings and preferences

*) Add constants for "magic" numbers to Settings, use them where
   applicable
*) Let the main extension file handle retrieval of settings
*) Change slider appearance setting to boolean type