When browsing the root directory, the application home button (on the
top left corner) becomes a drawer icon. When not in the root dir, the
icon is an arrow as usual.

This will increase the visibility of the document providers feature.

Also, modified the behavior of the system back key to close the drawer
in case it is open.

Change-Id: Id0ce932907bcec4b8601029482eda0c1c1f0df35
Reviewed-on: https://gerrit.libreoffice.org/16246Reviewed-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>
Tested-by: Jacobo Aragunde Pérez <jaragunde@igalia.com>