BaseTheme: remove unused properties

Reason for the change

There are two commits here, one to do some technical cleanup that's not really noteworthy, and another one to make hardcoded sizes instead use multiples of standard Kirigami units. The advantage here is that they can adjust with the font size if the user changes anything about their fonts, keeping the same relative proportions of objects in the UI.

Test plan

Change the font or font size and see that minimum window sizes, grid delegate sizes, etc ads subtly different.