Improve "Unchanged String" view
* Fixed styling of first row of results
* Make column headers visible even when a component is filtered
* Added dir and lang attributes to rows

"Unchanged strings" model:
* Don't try to compare non existing strings

Fixed undefined var $target_string2 warning in views/results_entities.php
Use htmlentities on search strings before displaying them, improve CSS for no results
Merge pull request #398 from flodolo/issue397

Encode HTML on search string before display, improve CSS without results
Merge pull request #395 from flodolo/fix_unchanged

Improve "Unchanged String" view