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/frontend-plugins/frontend-tools/forms/output-timer.ui

@@ -201,6 +201,13 @@
           </property>
         </widget>
       </item>
+      <item row="6" column="8">
+        <widget class="QDialogButtonBox" name="buttonBox">
+          <property name="standardButtons">
+            <set>QDialogButtonBox::Close</set>
+          </property>
+        </widget>
+      </item>
     </layout>
   </widget>
   <resources />

UI/frontend-plugins/frontend-tools/output-timer.cpp

@@ -22,6 +22,8 @@ OutputTimer::OutputTimer(QWidget *parent)
 		SLOT(StreamingTimerButton()));
 	QObject::connect(ui->outputTimerRecord, SIGNAL(clicked()), this,
 		SLOT(RecordingTimerButton()));
+	QObject::connect(ui->buttonBox->button(QDialogButtonBox::Close),
+		SIGNAL(clicked()), this, SLOT(hide()));
 
 	streamingTimer = new QTimer(this);
 	streamingTimerDisplay = new QTimer(this);