window: Don't override tile monitor
Commit 033f0d11 added a fallback in case the tile monitor wasn't set before, but didn't actually check for a previously set value. As a result, the "fallback" is not set unconditionally, which may differ from the expected monitor: The tile monitor is determined by the pointer position, while the window's monitor is the one where the biggest part of the window resides on.
Fixes #1389 (closed)