applets/weather: Optionally show the temperature in a badge overlay

Add an option to show the temperature over the weather icon.

We leverage the recent BadgeOverlay component to add useful info without requiring more space, which can be important, specially in thicker panels or the system tray.

Use a new configuration key along the previous one, so existing configurations are not affected and no migration code is needed.

Compact Representation	Config Options

Depends on !267 (merged) (first commit for easier testing)