fix nonmodal dialogs going behind main window in linux

notes:
was okay in windows
wnd position is not preserved on hiding via Close event now, requires extra handling, as in class RotateDialog or g_guiSettings (no much critical cases exist)
min max buttons aren't shown (when requested) in Ubuntu, shown in Windows; were shown for Qt::Window flag

radiant/groupdialog.cpp

@@ -84,7 +84,7 @@ GroupDlg::GroupDlg() : m_window( 0 ){
 void GroupDlg::Create( QWidget* parent ){
 	ASSERT_MESSAGE( m_window == 0, "dialog already created" );
 
-	m_window = new QWidget( parent, Qt::Window );
+	m_window = new QWidget( parent, Qt::Dialog | Qt::WindowMinimizeButtonHint | Qt::WindowMaximizeButtonHint | Qt::WindowCloseButtonHint );
 	m_window->setWindowTitle( "Entities" );
 
 //.	window_connect_focus_in_clear_focus_widget( m_window );