QTimer: add convenience singleShot methods for functors

This brings QTimer::singleShot on par with QObject::connect in
terms of the new Qt5 syntax. With this patch, it is now possible
to connect singleShot to a member pointer, a static function
pointer and a functor (with or without a context object).

The short code path for 0 msec is not yet implemented - it will
require further modifications to QMetaObject before it will be.

An additional SFINAE on the new singleShot overloads had to be
implemented to prevent tricking the compiler into believing
const char * might be a function pointer.

[ChangeLog][QtCore][QTimer] Implemented new style connect syntax,
including functors, in QTimer::singleShot

Task-number: QTBUG-26406
Change-Id: I31b2fa2c8369648030ec80b12e3ae10b92eb28b4
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>