snippets: look for last occurrence of _ for instance name.
snippets: allow multiple expansions within a tab-stop

You can now have multiple expansions within a tab-stop for a snippet.
This can be done using `` to make sub-expressions.

For example:

  ${3:Hello `$1|capitalize` `$3|upper`}

The whole region will be selected when moving to that tab-stop.
snippets: use new `` syntax to define defaults for g_return_if_fail().

The "fail" snippet, which expands a g_return_if_fail() check, can now
use the current filename to guess the default check. Since most functions
happen to perform this pre-conditional, it saves a bit of typing.