Release 1.6.0

• Update wayland.xml to latest
• XDG stable: use mock methods instead of notification lists
• Test popup constraint_adjustment
• Tests for XDG shell (stable) version 5
• Test that text input is entered after child window is closed
• Copy geometry headers from Mir
• Test popups are dismissed in the correct order
• Test buffer can be deleted after it's attached
• Less flaky synchronization for VirtualPointerV1Test (#297, #294)
• VirtualPointerV1Test: no-events-sent test fails if events are sent (#296)
• Fix various frame-event misunderstandings
• Change remaining std::experimental::optionals to std::optional