Add icons for main menu buttons, ability for buttons in general

The current interface is almost exclusively in written English.
This makes usage of the program more difficult for people who can't
distinguish English words, or remember specific VR/English technical
terminology.

Adding small icons to main menu items will make it easier for the
aforementioned groups to use the application.

This also adds the ability for buttons in general to have a custom icon,
if necessary.

src/res/qml/common/MyPushButton.qml

@@ -3,6 +3,10 @@ import QtQuick.Controls 2.0
 import "." // QTBUG-34418, singletons require explicit import to load qmldir file
 
 Button {
+    // Overload this
+    // Empty string = no icon
+    property string iconPath: ""
+
     property bool activationSoundEnabled: true
 	hoverEnabled: true
 	contentItem: MyText {
@@ -15,6 +19,12 @@ Button {
         color: parent.down ? "#406288" : (parent.activeFocus ? "#365473" : "#2c435d")
     }
 
+    Image {
+        source: iconPath
+        height: parent.height
+        width: height
+    }
+
     onHoveredChanged: {
         if (hovered) {
             forceActiveFocus()