added preliminary support for displaying images

Ignore-this: 127a0f3b142b6fa31ebe45733a154070

darcs-hash:20140119205022-c21fe-41357c92f42a01b645f25433b13741b6713849bd
fixed some memory leaks in the lua api

Ignore-this: e03a12a2365898c977942dabc9e2cff3

widgets not placed on canvas were leaked.

darcs-hash:20140120062011-c21fe-5913a1b6bb0c295bf104177db3c6daa61c439510