Share map only support one selected object, but not multiselect.

[leafisfalling]

Describe the bug
When I use "marking in map" tool to select several objects and then go to share the map, the URL created just contains one selected object instead of all the selected object in the shared map.

To Reproduce
Steps to reproduce the behavior:

1. Activate 'Marking in map' tool and select a few objects
2. Go to Share map to create a URL
3. Open the URL in a new browser window
4. Then it can be seen that no matter how many objects selected there is just one shown in the shared map.