added ability to set the visibility of widgets in the frontend

Ignore-this: d7e918c18d0b480cb96376bedc92c3ca

darcs-hash:20141201144507-c21fe-19d6d3c74cefc3bb10feb761d3091b12ee259c48
added visible,show and hide to API

Ignore-this: 246f607ad886296a8684968ba5f51ff3

Now you can show and hide widgets in an app.

darcs-hash:20141201144621-c21fe-60fe2406c035d8bf55453c53d2b49ddf7e21b24e
Fixed a bug in clearing a list.

Ignore-this: e3faa687f80b90bcf1031aa5c3e13cd1

The widgets in a list were still visible after clearing the list.
Now all the widgets are hidden.

You can still show the widget again if you have a reference to it on the lua side.

darcs-hash:20141201144721-c21fe-810f6fb1a103d0c6087caf334ab319ffd4f3ea98
fixed bugs in app editor when code is re-run

Ignore-this: 6919ec30b334a9eb674f196a69d7e1b8

The lua state was not cleared when the code in the app editor was re-run.
This caused some ghosts and phantoms to appear.

darcs-hash:20141201152643-c21fe-81c3c308805f32610cf9b4fd15677696f96e87b7