Add field "Folder" to theme content items and to message tool tip

Summary:
Add possibility to add the messages account/folder as a theme content
item. Additionally, show the path in the messages tool tip.

FEATURE: 420644
BUG: 420644
FIXED-IN: 5.15.0

Test Plan:
New tool tip:

New "Folder" item in content items

Message list:

Reviewers: VDG, dvratil

Reviewed By: dvratil

Subscribers: dvratil, kde-pim

Tags: KDE PIM, KDE PIM: KMail

Differential Revision: https://phabricator.kde.org/D29602