QtAutogen: Don't modify target source files while iterating them.

Populate a separate vector of files and append them separately. This
was the pattern used prior to commit 035b6908 (Autogen: Split AutoRcc
handling into two methods, 2013-12-10), which was erroneously not
maintained in that refactoring.
Merge topic 'fix-Qt-rcc-file-handling' into next

870bd16 QtAutogen: Don't modify target source files while iterating them.