UI: Add checkbox for visibility of new scene items

Note: This functionality should probably be considered as a temporary
solution.  Ideally, sources should not be visible on creation until the
user presses "OK" in the initial properties dialog.  Also, the problem
of initial visibility can additionally be solved by implementing the
preview/program functionality.  Once preview/program is implemented the
need for this patch will be negated, and the option should probably be
removed.

Closes jp9000/obs-studio#483

obs/forms/OBSBasicSourceSelect.ui

@@ -49,6 +49,16 @@
        </property>
       </widget>
      </item>
+     <item>
+      <widget class="QCheckBox" name="sourceVisible">
+       <property name="text">
+        <string>Basic.SourceSelect.AddVisible</string>
+       </property>
+       <property name="checked">
+        <bool>true</bool>
+       </property>
+      </widget>
+     </item>
     </layout>
    </item>
    <item>