feature request: Load offline maps

#### Issue details
Is there a feature implemented for loading offline maps? If not, I recomment to add this feature. It makes the mission planning easier if you are at the test-site without a stable wifi connection.

best regards

The issue names no files, tests, or entry points. First inspect Mission Planner's existing map-loading behavior and clarify the required offline-map sources, storage, and coverage. Done would mean maps can be loaded and used for mission planning without a stable Wi-Fi connection.

Written by the indexing model from the issue text.