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
• 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