HaikuDepot TextView: add support for hyperlinks/clikable areas

Specific text spans can be assigned a cursor and BMessage to send when
they are clicked. This allows for implementing hyperlinks, specific
popup menus, and clickable text of any type.

With some extra work it can also be used to implement spell checking
suggestions, buttons, and so on.

Change-Id: I390e0c44656da76a950c432bdd934bd51af49baf
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7130
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

Renamed HaikuDepot's home to haikudepot.

* This closes #10699. I mentioned that to stippi after his first commit
of HaikuDepot, anyway :-)

Renamed HaikuDepot's home to haikudepot.

* This closes #10699. I mentioned that to stippi after his first commit
of HaikuDepot, anyway :-)