Add utility text function KShell::tildeCollapse

Summary: KShell::tildeCollapse return a string with homedir (usually /home/$USER) replaced by ~.

Test Plan: ctest

Reviewers: Frameworks, broulik, ngraham, ervin, apol

Reviewed By: ngraham, ervin, apol

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26122