Show valid subprocess CWD for %D tab title format

Summary:
`%D` did work only with main process - for every subprocess it just
displayed "-". With the patch "-" is displayed only in processes for
which CWD can't be read (e.g. root processes).

Test Plan:
* Use `%D` in tab title format
* Run `vim`
* (optionally) run `:cd /usr` in vim

**Actual result:** "-" is shown in the title instead of `%D`
**Expected result:** valid CWD is shown in the title instead of `%D`

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: hindenburg, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D17865