Made symbol searching plug-able through indexing support.

The indexing support for the built-in code model is moved to its own
file. Symbol searching will now call for a searcher through that support
interface, which will create a fully configured and ready-to-go searcher
that can be started in the/a future.

Change-Id: Idc3ee1c7c789a69fa05ee1d42415313dcea94cf8
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>