applets/keyboardlayout: fix bugs in display styles

This commit fixes the following bugs:

1. The text preview is no longer unconditionally made uppercase, so now
   it actually matches what will be displayed in the applet.
2. The text preview no longer always displays the short name even if the
   user set a custom display name, so now it actually matches what will
   be displayed in the applet.
3. When the current layout has no flag defined, the flag preview now
   displays a "no flag available" placeholder warning text, rather than
   nothing at all, which made the UI look like something was missing (in
   fact, there was!).