UI: Make sure all dialogs have close buttons

This adds close buttons to remux dialog, output timer dialog, and
advanced audio properties dialog.  I also did a small refactor of the
remux dialog so the buttons were consistent with other dialogs.

Closes jp9000/obs-studio#876

UI/window-basic-adv-audio.cpp

@@ -1,6 +1,8 @@
 #include <QVBoxLayout>
+#include <QHBoxLayout>
 #include <QGridLayout>
 #include <QScrollArea>
+#include <QPushButton>
 #include <QLabel>
 #include "window-basic-adv-audio.hpp"
 #include "window-basic-main.hpp"
@@ -65,11 +67,20 @@ OBSBasicAdvAudio::OBSBasicAdvAudio(QWidget *parent)
 	scrollArea->setWidget(widget);
 	scrollArea->setWidgetResizable(true);
 
+	QPushButton *closeButton = new QPushButton(QTStr("Close"));
+
+	QHBoxLayout *buttonLayout = new QHBoxLayout;
+	buttonLayout->addStretch();
+	buttonLayout->addWidget(closeButton);
+
 	vlayout = new QVBoxLayout;
 	vlayout->setContentsMargins(11, 11, 11, 11);
 	vlayout->addWidget(scrollArea);
+	vlayout->addLayout(buttonLayout);
 	setLayout(vlayout);
 
+	connect(closeButton, &QPushButton::clicked, [this] () {close();});
+
 	installEventFilter(CreateShortcutFilter());
 
 	/* enum user scene/sources */