This new setting, located in the layout item "Rendering" section,
allows users to set an optional "group name" for use in GeoPDF
exports. When set, a matching layer tree group will be created
in the exported GeoPDF and the item will only be visible when
this group is checked.
This allows content to be selectively displayed as a group by
viewers of the GeoPDF. Eg, it can allow extra layout content
such as descriptive labels or legends to only be shown when
layers from the group are visible, making GeoPDF export much
more flexible.
sip6 converts all enums to python Enums, but ONLY creates
Enums with IntFlags types when the c++ type is an enum class : int.
Accordingly we need to patch back in all the operations which treat
enum values as ints, like |, &, bool, etc.
The long term solution here is to move all our c++ enums to enum
class, but that's not always straightforward and can break API
for plugins if it involves the signature of virtual methods.
