elm_test: Add a test case for gfx filters

elm_test: Add a test case for gfx filters

This adds a test window for the efl_gfx_filter (also known as
evas filters). The builtin examples are not incredibly pretty
but they showcase some of the most basic features. They also
show that some of the filters need to be improved.

Note that the code contains a lot of FIXME due to the incompleteness
of our EO API. Also, Efl.Ui.Text still has quite a few issues (sizing,
lack of a working change event, ...) so the UI doesn't even look as
designed (a label is missing). Hopefully this should get fixed over
time.

Oh and this is mostly using EO APIs but that's a terrible idea as
evas_object_text is not going to be exposed in EO land... except
it's the only object implementing filter support (with image).