Can a PhD supervisor take unpublished ideas from a student's dissertation?
A PhD dissertation is published work, not a private document. It is publicly available to anyone with a library access (via interlibrary loan or via an appropriate dissertation vendor).

Of course, if anyone (including your former PhD supervisor) uses results from the dissertation, they must include an appropriate citation. That way you get a credit for your contribution. 

If your contribution is considered valuable by the standards of your field, you can write it up and publish it in a refereed journal for better visibility (compared to the dissertation). You can also upload your thesis on an institutional repository or arXiv.

My advice: do not waste time arguing about ownership of ideas. It will not help your career. You had one idea that was good enough to interest people; make more!