[EPIC] Basic visual editor functionality

[alex4401]

• [x] #65

The visual editor should allow:

• [ ] Changing a group's collectability status
• [ ] Changing whether a group is shown by default
• [x] Adding markers to the map
• [x] Name markers, set description, image
• [x] Edit markers
• [x] View coordinates regardless of map and instance settings, preferably after a click on the map

[alex4401]

This is being developed on the mainline (v0.14.0) and will be backported to v0.13.5.