Do not try to highlight empty string

Signed-off-by: Michal Čihař <michal@cihar.com>
Extended search possibilites.

It is now possible to combine filter with searching.

Fixes #550

Signed-off-by: Michal Čihař <michal@cihar.com>
Allow to search translated strings only

Signed-off-by: Michal Čihař <michal@cihar.com>