Simplify AbstractClient::applyWindowRules()

Trying to reconstrain the geometry after applying rules is unnecessary
because if the geometry is forced using a rule, resizeWithChecks()
should not change it; if the window is maximized or has entered fullscreen
mode, window gravity is irrelevant. If the window is shaded, this
resizeWithChecks() is not needed.

This change simplifies AbstractClient::applyWindowRules(), which allows
to split adjustedSize() to further simplify geometry handling.