Locator: Case sensitivity of input affects prioritizing

So far candidates were prefix matched case sensitive which led to
an unfavorable results order.

With this patch, if the input is lower case, the prioritizing happens by
a case insensitive prefix match. Otherwise the match happens case
sensitive (just like before).

Example:

    Search for e.g. "m cppmodelmanager"
    Top result before: AbstractEditorSupport (match at parameter type)
    Top result now: CppModelManager

Change-Id: Ic27042cfe717be812a2237a3437399597c98dd74
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: David Schulz <david.schulz@digia.com>