Include the Plasma Vault name in the mount dialog title

Summary: Allows synergy with password managers that rely on the window title in order to supply the appropriate password.

Test Plan: Compiles and runs as expected both on master and Plasma/5.18 branches.

Reviewers: Plasma, davidedmundson, ivan, VDG, ngraham

Reviewed By: Plasma, davidedmundson, VDG, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28914