untangle the egl-x11 relationship in the build system

egl-x11 is used in two places:
- the eglfs-x11 plugin, which has a hard dependency on xcb-xlib
- the xcb-egl plugin, which has a soft dependency on xcb-xlib

that means that the egl-x11 configure test needs to be untangled from
xcb, and that eglfs-x11 should be a separate feature with a proper
dependency declaration.

when the plugins that need egl-x11 are not built, it also makes no sense
to build the respective integration in the egl_support module (even if
it's possible to coax it into building), so adjust things accordingly.

Change-Id: Ic729d0b7c893dd00844567329205c24ea2703033
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

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