applets/keyboardlayout: fix bugs in display styles

This merge request is an alternative to !1073 (merged), which attempts to salvage the existing UI by fixing its bugs. It fixes the following bugs, all of which can be verified visually in the before/after images.

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!).

	Before	After
My custom English keyboard layout
Greek

cc @butirsky @teams/vdg @teams/usability