Proper fix for KeePass freezing the window manager under Mono

This commit implements a full and working fix for KeePass freezing the
window manager under Mono.

The core problem is that under Mono, clipboard manipulation requires the
Mono UI thread to continue processing it's message pump while other
clipboard manipulation is happening, else xsel and related apps will block.

This patch adds the DoEventsWorkAround AppRunFlag, which causes
NativeLib.RunConsoleApp() to asynchronously run a process, and execute
DoEvents while it's running - allowing clipboard manipulation to succeed.

While DoEvents is running, user input is disabled to avoid problems with
possibly re-entrant functions.