New Tabswitcher: Fix cut-off path strings

Summary:
...and add GUI test program for testing the TabswitcherFilesModel.

The reason was, that the truncating function used the Kate document name which can be "README.md (2)" where the " (2)" is appended to the filename. In these cases 3 characters were cut-off too much (see https://phabricator.kde.org/D16054).

Reviewers: #kate, dhaumann

Reviewed By: #kate, dhaumann

Subscribers: dhaumann, cullmann, kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D16214