Support for multiple embedded maps in one page with frontendapi

[tha-nordiq]

Description

There is a limitation in the current system for embedding maps.
The embed-api does not allow multi maps on the same page. i have found 4 distinct problems that all would need to be solved for this to be possible.

1. the Openlayers map has a hardcoded id.
2. the hookRegister uses global hook registry causing leakages between maps
3. the LoadAfterTheme option only works for one of the maps, because the theme is rightfully only loaded once
4. the api is aimed at single map usage ie MapStore2.onAction

What kind of improvement you want to add? (check one with "x", remove the others)

• Minor changes to existing features
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Other useful information

My plan is for 1, 2 and 3 to open small improvement-pull request. That will be of general benefit.
4 is a bigger thing. Im instead working on a custom solution that might be converted into a pull request.

i would be very interested to hear if any work on embedded maps are currently happening or there are any plans?

[tha-nordiq]

Found another issue:
Reactdnd HTML backend is currently global.
should also be easy to fix