Allow parts (like kmail) to set a custom caption in guiActivateEvent

Test Plan:
Switching between korganizer and kmail in kontact
Now (together with a commit enabling the code in kmail_part.cpp)
kmail shows the path to the current mail folder in the caption.

Reviewers: mlaurent, winterz

Reviewed By: mlaurent, winterz

Subscribers: winterz, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D21625