Fix crash with new Problems Highlighter

This commit slightly optimize highlight applying by skipping non-text documents. This also fixes crash caused by calling updateOpenedDocumentsHighlight() method without any opened documents. In this case documentController()->openDocuments() returns single (non-text) document with url like file:///tmp/kdevelop_QW2530.patch which has fatal bug: if we call isActive() method from this document the crash will happens.