Add mechanism to change icons in RTL mode

Add mechanism to change icons in RTL mode

When the cursor in RTL text, icons for "depth-increment" or
"layout-toggle Enumerate" look wrong.

To fix this, the lfun "bidi" is introduced. "bidi ltr func" behaves
like "func" in LTR text, but is unknown in RTL text. "bidi rtl" does
the opposite. This allows to add two icons, but only have one icon
available. When no document is available, only LTR is assumed.

To make this work, the handling of unknown functions in toolbar has
been changed so this these functions can change dynamically their
existence.

The icon themes `default', `oxygen' and `classic' have been updated
accordingly.

Fixes bug #4451.