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:
{F8321268}

New "Folder" item in content items
{F8307551}

Message list:
{F8321283}

Reviewers: #vdg, dvratil

Reviewed By: dvratil

Subscribers: dvratil, kde-pim

Tags: #kde_pim, #kde_pim_kmail

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