snippets: rename tab_size to tab_width to match gtksourceview.
snippets: respect insert-spaces-instead-of-tabs upon snippet insertion.
Revert "snippets: ensure GbSourceSnippet knows about tab_width."

This reverts commit 2725118cde1fa85cde936ed4653ab52ea71cef1e.

Conflicts:
	src/editor/gb-source-view.c
Revert "snippets: plumb if snippets should insert tabs."

This reverts commit a3925a193df0b2b67c1bf27bcb7809a0809b1eb8.

Conflicts:
	src/editor/gb-source-view.c