Keep the Select Lines dialog open when no lines match

[MawCeron]

Summary

• DialogSelection::Process closed the dialog unconditionally on OK, even when the search/action changed nothing (count == 0) — right after showing a message box saying so. That forced the user to reopen the dialog and redo the search from scratch after any typo or unchecked option.
• Only close the dialog when the selection actually changed; otherwise leave it open so the user can adjust the criteria in place.

(Reopens #679, closed only because the branch's commit needed its authorship fixed — same change, no functional difference.)

Fixes #349

Test plan

• Read the code path in src/dialog_selection.cpp to confirm count is 0 exactly when the message box reports no lines were set/added/removed, for every Action branch (SET/ADD/SUB/INTERSECT).
• Manual: built locally (meson/ninja, Linux/GTK3), opened Subtitle > Select Lines..., searched for text that matches nothing, pressed OK — the "Selection was set to no lines" message appears and the dialog now stays open instead of closing.

[CoffeeFlux]

If you could say more about your workflow on the issue itself that motivated this, that'd be much appreciated. I'm not clear whether this is a change worth making or something else related to this flow needs to be thought through, so more information would be helpful.