Add comments to confusing piece of code.
Return stolen focus when closing MainWindow.

On OS X (and maybe other platforms), CopyQ "steals" focus when it
is shown. This is desired. Unfortunately, when closing the window (e.g.
by pressing Escape), the focus is not returned to the previous window.
This change adds a new slot MainWindow::closeAndReturnFocus() which
does so.
Handle Qt "AnyMime" UTIs on OS X.
Handle URL MIME <-> UTI conversion ourselves.

This allows us to produce nicer URLs.
Add info about OS X to docs.
Store both the app and window titles on OS X.
Fix display of copied files on OS X.