Fix GrantleeTheme::ThemeManager being leaked.
When a Viewer was deleted, its ThemeManager was kept alive.
So it would still react to KDirWatch (when installing files) and it
would try to update a menu (QActionGroup) that was deleted with the
KMail, Enter to open separate viewer, Alt+F4, (uncommitted)
debug output shows that now the ThemeManager is deleted, while it wasn't before.
Reviewed By: mlaurent
Subscribers: KDE PIM
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D12431