Option to add multiple wms_endpoint layers as separate layers

[Stereo]

The wms_endpoint imagery things are fantastic.

When adding more than one, it would be nice to have the option to add all of those layers separately. That way, I can then toggle through them easily.

[Don-vip]

WMTS also requires this if we want to add Landsat from NASA servers, see #15940

[wiktorn]

In 13733/josm:

Imagery definition refactor

Extend imagery definitions by:

• allowing setting default layers for WMS_ENDPOINT and WMTS
• allowing setting minimum expires time for tile for this imagery
• allowing setting custom headers that will be sent for all requests

(get map, get capabilities) for this imagery

Additional changes in code:

• use TileJobOptions to pass miscellaneous options to loaders
• refactor WMSImagery to use SAX parser

See: #15981, #7953, #16224, #15940, #16249

[wiktorn]

In 13734/josm:

GUI for imagery definitions refactor

Extend preferences panel by:

• allowing setting default layers for WMS / WMTS
• allowing setting minimum expires (expert mode)
• allowing setting custom headers (expert mode)

See: #15981, #7953, #16224, #15940, #16249