Show progress bar when filtering using regex

[merces]

To reproduce:

1. Open a huge file. Let's say, a 50GB.
2. Press s.
3. Perss f and type a filter.
4. Press Enter.

dz6 start filtering the shown strings, but it take too long and the app remains stuck until it ends. User receives no feedback. We should at least show a progress bar to some time remaining for the operation to complete.