Template events are loaded incorrectly

After going through the template events test for my Twig branch, I realized they were designed incorrectly.

Only ONE template event file is supposed to be included from EACH
extension. As it was before, EVERY matching template event file from
each extension was included (this was how it was designed).

E.g.
Event call in prosilver "foo"
Extension has template "foo" in prosilver AND all

foo from all would be included, then foo from prosilver would be included

This was clearly not designed correctly as only the most specific event
file from each extension should be loaded, otherwise events could only
ever be put in a single style tree (either only all, or only prosilver
and subsilver2 and any style that inherits from neither of those).
Otherwise the events would be duplicated on output (which is clearly not
desirable).

This is fixed with the Twig PR, but should be noted in case something would happen that we cannot use Twig (which is unlikely).