windowswitchers: Add a placeholder message

Add a placehoder message when there is no windows in the model, in a similar way as the default thumbnail_grid ( kwin!5128 (merged)).

• 'compact' gets a slighly different implementation to match the visuals of the regular entries
• 'big-icons' also adds a small sizing fix which was not apparent without the placeholder

Sorry I forgot to switch language before screenshotting, but this shows I copy-pasted the i18n string correctly 😄

This hits a bit my DRY-OCD, but I don't know whether it's worth it to create a specific placeholder component specifically for this, as part of the tabbox QML API.

	Regular aspect with windows	No windows BEFORE	No windows AFTER
Big Icons
Compact
Sidebar
Flip/Cover