symbols: Adding IdeCodeIndexer & IdeCodeIndexEntry

IdeCodeIndexEntry represents an entry in code index.
IdeCodeIndexer interface has to be implemented by plugins
which want to provide indexing service for a language
source file. This interface will take file as input and
generates IdeCodeIndexEntries using which index entries
can be retrieved for that file.

https://bugzilla.gnome.org/show_bug.cgi?id=786700