[PATCH] Imagery definitions refactor

[wiktorn]

Attached patch extends imagery definitions with:

• ability to define default layers for WMS_ENDPOINT and WMTS
• ability to set custom headers from GUI
• ability to set imagery as properly georeferenced

Apart from that, with this patch comes new WMS GetCapabilities parser based on SAX.

There are also easily separated fixes for CachedFile that I came across when developing this changes:

• cache validity check
• too long paths on Windows

The long vision is to move all (or all except some crippled imagery providers that do not provide GetCapabilities service) WMS imagery definitions to WMS_ENDPOINT type.

Features yet to be developed:

• add edit option in ImageryProvidersPanel which will open prefilled GUI as when adding
• add context menu in Layers window to change layer if we use WMS_ENDPOINT / WMTS
• add "copy as maps entry" which would allow to easily add new entry in josm.openstreetmap.de/maps based on user imageries
• add "copy as string" which would allow easy share of entries within community

As this is bigger change that is hard to split, any testers are welcome.

For easier review - I've created pull request on my GitHub:
​https://github.com/wiktorn/josm/pull/1

You may comment there or here.

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