qdoc: Added 2nd argument to \generate list classes command

The qdoc command \generatelist has an argument that tells
qdoc which list to generate. When the argument is "classes"
qdoc generates the "All C++ Classes" list. qdoc looks for a
common prefix for all the members of the list and sorts the
members of the list using the character that follows the
common prefix. The problem was that the common prefix "Q"
was hardcoded in qdoc.

This update allows the \generate list command to have a
second argument, which is the common prefix. If the common
prefix is not provided, qdoc sorts the members of the list
on the first character.

There is only one use of the \generatelist command with
the classes argument in Qt5. It is changed to be
\generatelist classes Q, and the qdoc user manual is
updated to reflect this change.

Task-number: QTBUG-38226
Change-Id: Icc556efa517501f5c95d36db0ca35977af76f664
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>