register_widget_type() can now be called in any order.
Combined with the addition of void *p to widget_t::p, the new version of
register_widget_type() allows new widget types to be created by xuni
applications. (The new version of register_widget_type() simply performs
a binary insertion sort to place the new widget type into the correct
place.)
Note that a hard-coded value will still have to be created for each
widget type at the moment, which could involve conflicts as different
xuni plugins pick the same number, say, 100.
The union widget_t::p ought to be made into a void pointer. Then widget
types could be truly modular.
Added a define, README_FILE, to replace the hard-coded string "README".