qt: Clean up foreground window code
* qt/main.cpp (ForeignWidget): Remove. This did nothing with Qt 5. (setup_foreground_window): New helper to set parent and window flags. (qt_cmd_handler): Use setup_foreground_window. * qt/pinentrydialog.cpp: Disable Windows API hacks.
This should hopefully work a bit cleaner then the old code.
Setting us as transient to the Desktop is the same what
Gtk 2 does.