emojier: Don't coerce user to void

Annotate return type of a QML method as well

Otherwise QML engine insists that "user should be coerced to void". Don't do that to our dear users.

emojier: Clean up QML in many ways

such as:

• Drop QML import versions;
• Fix up code style and whitespaces;
• Explicitly specify arguments in signal handlers
• Add types to QML methods

Stretch GlobalDrawer's custom heading, so it can elide and work with RTL

Kirigami.AbstractApplicationHeader would vertically center all of its children, so we only need to provide left and right anchor.