Feat: emit mapmounted event

[RobertOrthofer]

Implemented changes

With this PR a CustomEvent mapmounted is fired after the map is mounted. This is the earliest possibility i have found to emit a useful event, since I don't think an event inside the constructor of the EOxMap can realistically be listened to.
The detail of the event is the openlayers-map, the targetElement of the map should always be defined at that point.

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closes #914

[StefanBrand]

Looks like a straightforward solution. 👍

Adding @viktoraronfarkas for information