Platform: more documentation updates.
package: build XEglApplication on non-desktop ES.
Platform: error out when trying to use NaClApplication outside of NaCl.
Platform: move eglErrorString() helper to separate header.

It will be useful also elsewhere.
Platform: undef even more Xlib nonsense.

Do I really need to have everything uniquely prefixed with
uncomprehensible names to defend myself from headers that were made by
careless people two centuries ago?! Rage level +37.