Bug 740059 - Text tool changes from "Dynamic" to "Fixed"...

after text box moved using Alt key

In gimp_text_tool_button_release(), handle the "moving" case
separately instead of running into the default else branch that
assumes the user clicked outside and made a new text layer.

(cherry picked from commit 293e5bde9e478c489ac6396e67cdd1782427414c)