Instead of making yas-expand and yas-expand-from-keymap search for the
command that would have been called, use a conditional keybinding so the
Emacs' builtin keybinding lookup code will do the searching instead.
* doc/faq.org: Remove section about old method binding method. Update
example to use new method.
* doc/snippet-expansion.org (Trigger key): Update explanation for new
method.
* yasnippet.el (yas-fallback-behavior): Mark obsolete.
(yas--maybe-expand-key-filter): New function.
(yas-maybe-expand): New conditional binding.
(yas-minor-mode-map): Bind it to TAB and <tab>.
(yas--maybe-expand-from-keymap-filter): New function, extracted from
`yas-expand-from-keymap'.
(yas-maybe-expand-from-keymap): New conditional binding.
* yasnippet-tests.el (yas--key-binding): New function, like
`key-binding' but overrides `this-command-keys-vector'.
(snippet-load-uuid): Use it.
(test-yas-tab-binding, test-yas-in-org): Insert snippet key before
testing binding.
