client: Highest sender prefix mode, migrate, fix

Add a "Highest mode" for showing sender prefixes, to only show the
highest in a given situation.  This mimics most IRC clients,
including Quasseldroid, while still saving the full mode details
behind the scenes.

Switch the UI from a Checkbox to a ComboBox, with all the 'fun' of
enums in Qt.  (Tip: don't code when tired.)

Add settings migration to preserve "None" for people upgrading, so
they won't suddenly see changes.

NOTE:  0.13 master users will need to opt in to enable showing sender
prefixes again.  A migration could be added, but the checkbox has
never been in beta or stable...

Fix up the "enableSenderBrackets" fallback default to be false, as
per Quassel itself.

Fix the sender prefix feature check to use "SenderPrefixes" instead
of "CoreSideHighlights".