Allow extensions control post buttons displaying

Currently post buttons list displaying is controlled by condition:

<!-- IF postrow.U_EDIT or postrow.U_DELETE or postrow.U_REPORT or postrow.U_WARN or postrow.U_INFO or postrow.U_QUOTE -->

This causes any custom post button added by extension using viewtopic_body_post_buttons_after event is dependent on the same condition and can't use its own condition to control self displaying.
The fix would be introducing template var which can be [re]assigned by extensions (f.e. via the viewtopic_body_post_buttons_list_before event) to allow adding their own conditions for the post buttons displaying.

prosilver style is affected.