add ACTION_DISPATCH event

The 'ACTION_DISPATCH' event[1] is triggered prior to any processing of the $ACT
variable[2]. This gives custom content the opportunity to add new "do" commands
and to process any custom form data.

This event is advisory only (there are no '_before' or '_after' advise)

Preventing default action will skip Dokuwiki's own processing of the $ACT value, this
includes its cleansing of the $ACT variable.

[1] also refer ACTION_TEMPLATE event
[2] $ACT is the do variable from the query string

darcs-hash:20060414195601-9b6ab-8ae62d2c3a7c6f8c09c77436d9adbaba5404f51b.gz