applets/calc: don't freeze Plasma & don't auto-resize desktop widget

When the fullRepresentation is used directly in a very wide panel, especially if other widgets are also present, Plasma may freeze as it gets stuck on switching between the representations - the compactRepresentation is large enough that it wants to switch, but the fullRepresentation ends up too small and switches back, forever.

Unsetting Layout minimum sizes in those cases prevents the issue from happening. We get rid of the preferred sizes completely, in favor of setting a default size on the PlasmoidItem - this way we still get the same initial size, but the content will not auto-resize on plasmashell load and potentially disrupt users desktop widget arrangements.