Add KP-Index display in MP

#### Issue details
Feature Request:
Add (optional) panel to MP, displaying current/ live KP-index values.

Ideally, this also displays KP warning/ watches data state. (Thresholds can be found in last link (Details tab).)
Also could add a visual indicator around that panel: green/ yellow/ red, depending on the warning state derived from the data.
Refresh period should be configurable in an options section - and/or add a "Refresh" button to the panel.

If MP is offline, revert to displaying last acquired gif image, with date over-layed (to be able to judge the validity of the data).

This feature would further increase the usefulness of Mission Planner as a goto tool in the field - one less thing to deal with for the pilot.

Possible data sources:
- https://www.gfz-potsdam.de/en/kp-index/
sample: http://www-app3.gfz-potsdam.de/kp_index/ql_bar.gif
- http://isgi.unistra.fr/
- https://www.swpc.noaa.gov/products/planetary-k-index
sample: https://services.swpc.noaa.gov/images/planetary-k-index.gif?time=1550147402000
(Info from details tab: "This chart updates every minute. The Estimated 3-hour Planetary Kp-index is derived at the NOAA Space Weather Prediction Center using data from the following ground-based magnetometers...")

#### Version
1.3.62
#### Platform
[X] All

No implementation files or tests are named. Start by reviewing Mission Planner's existing panel and options entry points, then compare the GFZ, ISGI, and NOAA data sources listed in the issue. Done means an agreed design and implementation for live KP-index values, warning-state visualization, refresh behavior, and offline fallback.

Written by the indexing model from the issue text.