Done for writerfilter::dmapper::PropertyMap.

Added public functions getPropertyValue, bool isSet and Erase. The function
getPropertyValue returns boost::optional<PropertyMap::Property>,
where PropertyMap::Property is an alias for std::pair<PropertyIds,css::uno::Any>.

Fixed all resulting compilation errors. The functions which iterated the map
(ListLevel::GetCharStyleProperties, ListLevel::GetLevelProperties and
SectionPropertyMap::_ApplyProperties) have been written to either used
GetPropertyValues or not iterate over the PropertyMap's attributes.

The properties of the public member functions are now properly used
(Erase and Insert(...,false)).

Change-Id: Ibd90a4ad831c9b7d18b2a50d4aa4eb3f2610cff8
Reviewed-on: https://gerrit.libreoffice.org/10558Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>