elm - selection manager - fix string buffer handling and format types

elm - selection manager - fix string buffer handling and format types

this has 2 bugs really. 1 - the format masks for plain text included
markup which ... is wrong - right? that should have a mime type of its
own... either way it leads to plain text pasts being handled wrong in
wayland... so this fixes that

also the string buffers in wayland just were nothandled right. buffers
were raw binary buffers plus size,not strings, but they were strdup'd
etc. assuming nul termination. oh so wrong to do that. this also fixes
that as i fpound it while fixing the above format bug.

this fixes T6284

@fix