LibGUI+WindowServer: Initialize minimum window size to zero

And remove unnecessary workarounds to the old limit of {50, 50} and
the cautious but arbitrary limit of {1, 1} for other WindowTypes.
Null rects are already the default when calculating minimum window
size and are the least restrictive but valid value.

Also returns early during minimum size calculations for frameless
windows, and verifies against negative minimum sizes and failure to
disable widget min size before setting a minimum window size. Layout
automatically overrides this setting each relayout otherwise.

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