The setters of both filterRegExp and filterRegularExpression are
currently normal functions. This patch moves them to slots to make
them usable using the old syntax. This can be done since there are
already overloads for both of them so people using the new connect
syntax would have needed to use qOverload already therefore there is
no SIC.

[ChangeLog][QtCore][QSortFilterProxyModel] Setters of both the
filterRegExp and filterRegularExpression properties are now slots and
can be used with the old as well as the new syntax.

Change-Id: Id5cd9a50fa4a62e2bbd6bd665b44bd25a0402852
Fixes: QTBUG-18113
Reviewed-by: David Faure <david.faure@kdab.com>