frontend: Add renamed Qt UI components

This commit only contains Qt UI components that are self-contained,
i.e. the translation units only contain code for a single class or
interface and don't mix implementations.

frontend/components/MenuButton.cpp

@@ -0,0 +1,36 @@
+#include "MenuButton.hpp"
+
+#include <QKeyEvent>
+#include <QMouseEvent>
+
+#include "moc_MenuButton.cpp"
+
+void MenuButton::keyPressEvent(QKeyEvent *event)
+{
+	if (menu()) {
+		switch (event->key()) {
+		case Qt::Key_Enter:
+		case Qt::Key_Return:
+			emit clicked();
+			return;
+		case Qt::Key_Down:
+		case Qt::Key_Space:
+			showMenu();
+			return;
+		}
+	}
+
+	QPushButton::keyPressEvent(event);
+}
+
+void MenuButton::mousePressEvent(QMouseEvent *event)
+{
+	if (menu()) {
+		if (width() - event->pos().x() <= 30)
+			showMenu();
+		else
+			setDown(true);
+	} else {
+		QPushButton::mousePressEvent(event);
+	}
+}