ui: Use window decorations on dialog windows (issue #376)

* added SubWindow flag to QtWindow to either use window decorations or show window within main window
* fixes black borders in some Linux window managers

bug id = 376

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src/lib_gui/qt/window/QtMainWindow.cpp

@@ -262,6 +262,19 @@ void QtMainWindow::updateHistoryMenu(const std::vector<SearchMatch>& history)
 	setupHistoryMenu();
 }
 
+void QtMainWindow::setContentEnabled(bool enabled)
+{
+	foreach (QAction *action, menuBar()->actions())
+	{
+		action->setEnabled(enabled);
+	}
+
+	for (DockWidget& dock : m_dockWidgets)
+	{
+		dock.widget->setEnabled(enabled);
+	}
+}
+
 bool QtMainWindow::event(QEvent* event)
 {
 	if (event->type() == QEvent::WindowActivate)