Add a context menu in webmap

[jedrek0429]

Partially implements #9.

TODO:

• [ ] better accessibility
• [ ] disappear on dragmove
• [ ] right clicking on the contextmenu should bring up the browser contextmenu
• [x] replace the switch case with a simple map that "gets" the configured items
• [ ] fix the weird visual of the accessibility highlighting when hitting tab with the contextmenu open

[jedrek0429]

moved to PR body

> there's still a few things left to do; > 1. better accessibility > 2. disappear on dragmove > 3. right clicking on the contextmenu should bring up the browser contextmenu > 4. replace the switch case with a simple map that "gets" the configured items > 5. fix the weird visual of the accessibility highlighting when hitting tab with the contextmenu open > [...] > > i think this is important to copy over as well: https://github.com/JLyne/LiveAtlas/blob/master/src/components/map/MapContextMenu.vue#L198-L213 @granny