API: Add an Application Programming Interface page

This page lists the all the public functions, structures and enums
provided by libusb.
The HTML page is generated by Doxygen.
API: remove unused libusb_hotplug_callback struct

The structure libusb_hotplug_callback was declared (but not defined) and
never used in the public API.
Doxygen: fix a duplicate label

libusb/hotplug.c:46: warning: multiple use of section label 'intro'
while adding section, (first occurrence: libusb/core.c, line 79)

Use label "hotplug_intro" instead of "intro"
Doxygen: fix a duplicate label

libusb/io.c:46: warning: multiple use of section label 'intro' while
adding section, (first occurrence: libusb/core.c, line 79)

Use label "io_intro" instead of "intro"
Doxygen: upgrade doxygen.cfg.in file

Warning: Tag `XML_SCHEMA' at line 942 of file `doxygen.cfg' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
Warning: Tag `XML_DTD' at line 948 of file `doxygen.cfg' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"

I upgrade the doxygen.cfg.in using: "doxygen -u doxygen.cfg.in"

The configuration file changed from Doxyfile 1.5.3 to Doxyfile 1.8.8