Fixed: window content view was not inset from the frame

Previously, _CPStandardWindowView did not inset the content view rect
from the window frame, thus its contents could overlap the window frame.

Now the content view is correctly inset so that content view content
is clipped to the window frame, except for the corners. Because the
corners are round and the window frame is a background color, they
can still be overlapped. this will be fixed in a later commit.

Closes #1798