Settings upgrade logic, classic for old installs

Add "VersionMinor" to Settings for backwards/forwards-compatible
changes.  Existing installs without this setting are distinguished by
checking if other settings keys exist.

Add upgrade logic to QtUiApplication to preserve previous settings
when changing defaults.  This avoids unwanted surprises if someone
likes the defaults, doesn't save them, then upgrades their client.
Otherwise, Qt would only save settings when changed.

(Core currently does not need version-compatible settings migration
 logic, but it can be added in the future if needed.)

Add a method to settings to check if a key exists, used when checking
if a non-default value was chosen.

Upgrading settings to version 2 will preserve Quassel's UI for those
with the classic look, but new installs default to modern style.  In
the future, this can be used to implement any other default changes.