Reimplement properly bidi icons

Reimplement properly bidi icons

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

Instead of relying on the clumsy "bidi" lfun of 2898c335, this new
version relies on a new Toobar tag BidiItem that inserts an action
which can have two icons, depending on the direction of the paragraph
containing the caret (or of the direction of the UI when no file is
open).

The alternative icon has the same name as the original one, with a
"+rtl" string appended to the lfun string. The alternative icon is
only active if the file is found. The icon themes `default', `oxygen'
and `classic' have been updated accordingly.

Fixes bug #4451.