* snippets: Allow keybinding overloading of snippet-next-cursor.

This allows to use the same key as for inserting snippets, or plugins to
map something to the same keybinding (e.g. if they implement a similar facility).

* snippets: Remove cursor position at the end of constructs.

This is not consistently done for all languages, and hard to get right
e.g. for python. It's probably not terribly useful either.

* snippets: Use Scintilla indicators for cursor posititons
* api: Increment API version.
* snippets: restore behavior of cursor-less snippets
* snippts: use ascii character for the placeholder.

Do not require documents to be UTF-8 for using snippets.

* snippets: fix start/end detection, when searching for the next cursor

Tested @vfaronov