Replace COMMON_PAGE_FROMTEMPLATE with COMMON_PAGETPL_LOAD event

As discussed on the mailing list [1] this patch replaces the
COMMON_PAGE_FROMTEMPLATE with a more flexible event to better intercept page
template use.

Plugin authors need to change their plugins. Details on the event are
available at [2]

[1] http://www.freelists.org/post/dokuwiki/COMMON-PAGE-FROMTEMPLATE-event
[2] http://www.dokuwiki.org/devel:event:common_pagetpl_load