Revert "Basic multiseat configuration support"

This reverts commit 170b6a06a20c5c0d4b8d3b4a8b2bb5bb8cce485d.
[ui] .ui file updated with Glade 3.18
[ui] Added preserve_cursor wrapper to keep cursor on selected tree element when tree is changed; applied it in tree collapse and reload code
Added some comments
Added code to move treestore element with its subtree to another parent
Added multiseat tree mode, in which seat-attached devices are displayed as children of seat tree elements (their path starts with 'seat://' pseudo-URL sheme); device parent element discovery logic moved to get_device_parent_treeiter() function, which is multiseat-aware and does most of the work; devices which change seat attachment are moved in tree via the previously-added treestore_move() function
Tree element selection callback is now aware of non-device tree elements
Added current device menu with multiseat submenu for attaching devices to seats and creating new seats; its logic interacts with logind dbus interface
Added graphics to default subsystems (because graphics fb devices are master-of-seat devices)