UI: Add menu button (split button) class

When you assign a menu to a normal QPushButton, it becomes a button that
only allows you to have a menu.  This class lets you click the button
and have a menu at the same time.

obs/CMakeLists.txt

@@ -117,6 +117,7 @@ set(obs_SOURCES
 	window-remux.cpp
 	properties-view.cpp
 	focus-list.cpp
+	menu-button.cpp
 	double-slider.cpp
 	volume-control.cpp
 	adv-audio-control.cpp
@@ -159,6 +160,7 @@ set(obs_HEADERS
 	display-helpers.hpp
 	double-slider.hpp
 	focus-list.hpp
+	menu-button.hpp
 	mute-checkbox.hpp
 	volume-control.hpp
 	adv-audio-control.hpp

obs/menu-button.cpp

@@ -0,0 +1,33 @@
+#include <QMenu>
+#include <QKeyEvent>
+#include <QMouseEvent>
+#include "menu-button.hpp"
+
+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:
+			showMenu();
+			return;
+		}
+	}
+
+	QPushButton::keyPressEvent(event);
+}
+
+void MenuButton::mousePressEvent(QMouseEvent *event)
+{
+	if (menu()) {
+		if (width() - event->x() <= 15)
+			showMenu();
+		else
+			setDown(true);
+	} else {
+		QPushButton::mousePressEvent(event);
+	}
+}

obs/menu-button.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <QPushButton>
+
+class MenuButton : public QPushButton {
+	Q_OBJECT
+
+protected:
+	void mousePressEvent(QMouseEvent *event) override;
+	void keyPressEvent(QKeyEvent *event) override;
+
+public:
+	explicit inline MenuButton(QWidget *parent = nullptr)
+		: QPushButton(parent)
+	{
+	}
+
+	explicit inline MenuButton(const QString &text,
+			QWidget *parent = nullptr)
+		: QPushButton(text, parent)
+	{
+	}
+};