UI: Use a default size for the properties window

When I changed the properties window to remove the .ui file, I forgot to
give it a default initial size.  This just sets it to 720x580 by
default.

obs/window-basic-properties.cpp

@@ -55,6 +55,8 @@ OBSBasicProperties::OBSBasicProperties(QWidget *parent, OBSSource source_)
 
 	if (cx > 400 && cy > 400)
 		resize(cx, cy);
+	else
+		resize(720, 580);
 
 	QMetaObject::connectSlotsByName(this);