Limit folder panel to home directory if inside home

Summary:
Added the option to limit the displayed folders in the folder panel (F7) to the tree below the user's home directory if the current URL is inside the home directory.
This can be configured in the preferences General/Behaviour tab by checking the corresponding check box.

Reviewers: Dolphin, elvisangelaccio, emmanuelp

Reviewed By: Dolphin, elvisangelaccio, emmanuelp

Subscribers: emmanuelp, elvisangelaccio, Konqueror, Dolphin

Differential Revision: https://phabricator.kde.org/D7477