QStringList::join: add an overload taking a single QChar

QStringList::join: add an overload taking a single QChar

This overload avoids the needless heap allocation that the traditional
overload incurs due to the implicit QChar -> QString conversion
involved there.

In order to share the implementation between the two overloads,
QStringList_join now takes the separator as a (Char*,int) tuple
instead of as a QString.

Change-Id: I92961f13a3f19099de2a6e2df9f4789a12fc83a0
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>