Add a more reliable replacement for qt_add/removeObject().

These hooks only worked reliably with LD_PRELOAD on Linux/GCC, on other
platforms they depended on what exactly the compiler optimizer is doing
as well as some nasty assembler rewriting to actually access them. The
new system uses a simple array of function pointers that can be set to
custom hooks by tools that need this (based on ideas from Andre Poenitz).

This also covers qt_startup_hook (similar problem), and the Qt version
number that Andre had asked for.

Change-Id: I2c3e7950fd49b1b1d04176be34c2fff3293981b0
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>