[KFontChooser] Add new DisplayFlag; modify how flags are used

Summary:
Add a DisplayFlag to allow users of KFontChooser to disable showing the
checkbox that toggles listing only monospaced fonts. This is useful for
apps that embed KFontChooser, e.g. Konsole that has its own checkbox
and an accompanying "warning" button (in fact the checkbox in KFontChooser
was inspired by the one in Konsole, and we don't want the font widget
there to suffer by having two checkboxes doing the opposite function of
each other).

In ShowDifferences mode, call setEnabled(false) on the list views and co.
after checking "flags & ShowDifferences", using XOR "flags ^ ShowDifferences"
would fail if another flag was set.

Test Plan: See the kfontchooserdialogtest app

Reviewers: Frameworks, dfaure, cfeck, bport

Reviewed By: dfaure

Subscribers: kossebau, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29065