Agenda: Add new todo-unblocked and nottodo-unblocked skip conditions

Agenda: Add new todo-unblocked and nottodo-unblocked skip conditions

* lisp/org-agenda.el (org-agenda-skip-if, org-agenda-skip-if-todo):
Add new todo-unblocked and nottodo-unblocked skip conditions.  These
match as for todo and nottodo, but only for unblocked todo items.

TINYCHANGE

This patch adds two new tests to `org-agenda-skip-if': 'todo-unblocked
and 'nottodo-unblocked.  These match like 'todo and 'nottodo, but only on
unblocked todo items.  This type of test is useful when compiling custom
agenda views containing lists of currently actionable todo items.

Whilst it's possible to code such tests in `org-agenda-custom-commands'
directly (well, Elisp is Turing-complete: you can in principle code
anything!), it's far less convenient than a simple `org-agenda-skip-if'
test which can reuse much of the existing machinery.

Note that the attached patch applies on top of my other "Fix
org-agenda-skip-if bug" patch, though this new feature is independent of
that bug-fix.