x11-wm/plasma5-kwin: improve Wayland support

This is a backport / newly-developed patches for KDE Plasma
Wayland on FreeBSD. Some parts are upstream.

• Shuffle the USE_KDE line around a bit to make clear what the direct Wayland-dependencies are; also add the ones that were missing for full Wayland functionality.
• The major() and minor() macros on FreeBSD have "return type" int, while on Linux they are "unsigned int" (see makedev(3) for details). When passed to QVariant without a cast, that leads to type-mismatches in DBus messages.
• ConsoleKit returns a new seat, but that seat does not receive the libinput devices and never gets any input. Hard-code "seat0" since this codepath is Wayland-only.
• Fix build on i386 (but really, modern X11/Wayland desktop on i386 seems like a really, really unlikely combination).