pre_execute is never called on exposed forms plugins

While i was writing the patch for Exposed items per page, I have discovered that pre_render is never called for exposed forms plugins.

Also post_execute() is not necessary for a plugin, so I deleted it.

There is, however, a performance issue here. Since handlers are not loaded again when pre_execute is called into views_plugins_display, I had to add an init_handlers() call to determine if current view is using exposed forms. I don't know if it is really important to use a cache method or something similar.