Inject a $page_title variable to output the title tag. Alter themes to use it.
$theme->request->display_entry is now replaced with $theme->request->display_post, which uses some changes applied to Controller in 0.9.1 that allow display_entry and display_page both to set display_post to true as a generic case.  This is done with conditions in the rewrite rules.
Also keeps $post->title_title, which allows the title of a post to be filtered for output in the title separately from the output for display in-page, which is useful.
Fixes habari/habari#508