Simplify code for bidi icons

Simplify code for bidi icons

Most of the RtL icons that we provide are simply mirrors of the LtR ones.
Therefore let the code do that by default if no +rtl icon is found.

The only RtL icon left for now is for "layout Enumerate" (notice the
numbers in it).

Also remove useless getPixmap wrapper and several old bidi_xxx icons
that were forgotten in previous patch.