Help: Update Qt4 documentation.

Remove independent doc listing for Qt4ConfigurationSettings, Qt4Macros
and UseQt4. These are implementation details.

Recommend the use of IMPORTED targets.

Remove references to the UseQt4 file, which is obsolete in light of
the IMPORTED targets.

Add documentation for automatic tool invocation and options.

Remove reference to variables which should not be needed in modern
implementations.
Merge topic 'update-qt4-docs' into next

f81e67c6 Help: Update Qt4 documentation.