[KDirOperator] Middle-elide labels that are too long to fit

Summary:
Right now too-long labels get right-elided, which among other minor issues, causes
their filename extentions to not be visible. Dolphin is moving to middle-eliding
labels instead (See D19471), so this patch implements the same behavior for KDirOperator
so the file dialogs behave the same way.

CCBUG: 404955

Test Plan: See the Test Plan in D19471; same deal

Reviewers: #vdg, elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D23201