unstable/text-input: Add preedit text hints

There seems to be some demand from input methods to be able to style different parts of the preedit string differently. Since the reasons to do that are many, and vary between IMs and languages, there may not ever be a closed semantic list of hints.

But even though some IMs would much want to be able to specify colors, they lack the context to know what choice of colors provides enough contrast and legibility embedded on foreign UIs (e.g. accounting things like themes, accessibility, ...).

This MR meets at a middle ground, and provides a set of wildly generic semantic values, so that clients may theme these specifically, and the IM is able to provide some hints rather than none.

Control over underlining is given to the IMs, since that is less controversial.

Signed-off-by: Carlos Garnacho carlosg@gnome.org