It's a confusing name that comes from the ICCCM. The ICCCM is best
forgotten about.
With the addition of the potential new "transient" role meaning a
parent-relative toplevel like a long-lived popup, used for e.g.
tooltips, the set_transient_for name will become even more confusing.
This reverts commit 4c1a11074af2c2221d50b0c35d2d0d883647bc15.
Use the new window_set_transient_for / window_get_transient_for
and xdg-shell support for this...
Jasper St. Pierre
Kristian Høgsberg
Philip Withnall