Add an option to expand the widget in Desktop view and Panels

[BelkaDev]

Note for Latte-dock users: The concerned panel must be set to normal window (sys settings) to allow accepting the focus/input

[Zren]

A configuration key isn't persisted between restarts unless there's an entry in contents/config/main.xml.
https://develop.kde.org/docs/extend/plasma/widget/configuration/

Also, the logic is weird, so I'm guessing you didn't test this with plasmoidviewer. It'll default to not showing the full calendar on the desktop which is the current default behaviour.

	fillPanel=true	fillPanel=false
isDesktop=true	Full	Compact
isDesktop=false	Full	Compact

What we want:

	fillPanel=true	fillPanel=false
isDesktop=true	Full	Full
isDesktop=false	Full	Compact

[BelkaDev]

I think some people might keep the widget on their desktop, so I changed the initial intent which was meant for panels only

[BelkaDev]

Closing this PR as branches got mixed up
Also fixed the logic and it's now here