Fixed: a sheet's parent window did not become main

Sheets are like child windows, they can never become main. Previously, when an attempt was made to make a sheet the main window, its parent window did not become main as it should have. Thus the title bar would remain dimmed, even though the sheet was the key window.

With this commit, when a sheet is made main, it defers to the parent window.
Fixed: windows with sheets were filtering mouse move events.

Previously, if a window had a sheet, a mouse move event was passed to the sheet and then filtered. This disabled resize cursors for the parent window of a sheet.

This commit allows mouse move events to continue to the parent window so that resize cursors will work on the parent window as well.