wayland text input doesn't respect ibus preedit text attributes

The wayland text input by 62c97133 doesn't respect ibus preedit text attributes which could be set by ibus language engines. Currently the preedit text is always underlined.

For example we need something like this (gtk): https://github.com/ibus/ibus/blob/master/client/gtk2/ibusimcontext.c#L1515