Textboxes can be added to shapes via context menu -> Add Text Box since 126.96.36.199.
For reference, this commit added the context menu entry:
author Miklos Vajna <email@example.com> 2014-06-19 10:11:10 +0200
committer Miklos Vajna <firstname.lastname@example.org> 2014-06-19 11:29:31 +0200
sw: add UI for adding a textbox for a draw shape