mock: Refactor mocks and introduce filled cards

Now, mocks use a new component, MockCard, which encapsulates the functionality of the previous MockDesktop whilst also providing the card that was always used with the mocks, which they now fill rather than use padding.

Before	After

Test with: --pages Network,SimpleByDefault,Overview,KRunner,KDEConnect,Activities,Vaults

Additionally, the Networks page indicator has been fixed with a better approach. I'll pull this into a new commit if you want me to, Nate.