Add basic overmap

[snipercup]

This requires #14 to work. I will publish this PR as a draft to make sure #14 gets merged first

This adds a basic overmap that allows you to change to a different tactical map:

In-game you can press m to open/close the map. To change a level, click one of the red tiles. The properties of the tile are shown on the right side. This is just a mockup and will change in the future.

If you click on a non-red tile, it will tell you to select a valid target:

Some things that can be improved on in the future that are not included in this PR:

• The map seed changes when you switch level so you get a new overmap
• Saving/loading any map data
• Give the user better information about the location that's clicked
• The overmap tiles are placeholders and will be replaced with something more recognizable
• Improve performance while panning the overmap. Right now there's a small stutter when new chunks are added

[snipercup]

I didn't include the actual keybinding for m. I will include it in the next pr. So right now pressing m does nothing.