Feature Request: Enhancements to Plan > Map Tools > Prefetch

#### Enhancement details
I would like to see a couple of extra options on the Prefetch tool. Accessed via **Plan** screen, draw a range, right click, **Map Tools** > **Prefetch**.

Firstly, you can already choose the _max zoom_ level. However, an additional option to also choose the _min zoom level_ would also be great.

Secondly, and more important than the first feature, would be an option to ignore tiles that are already downloaded.

#### Version
I am using 1.3.74 build 1.3.7563.27684

#### Platform
[ ] All
[ ] AntennaTracker
[X] Copter
[X] Plane
[X] Rover
[X] Sub

Airframe type, hardware type, and logs are not applicable.

Start in Mission Planner's Plan screen and follow Map Tools > Prefetch, inspecting the existing max-zoom option and tile-download behavior. Done means the tool supports selecting both minimum and maximum zoom levels and can skip tiles that are already downloaded; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.