add WmsRasterOverlay support and UI

[corybarr]

Adds Web Map Service (WMS) support:

Test file: WmsTest.zip

Resolves #609

[corybarr]

@r-veenstra I guess on good max ranges on the zoom level. Can you see if they're reasonable?

[corybarr]

@r-veenstra I'm wondering if adding a "Refresh" button to the WMS raster overlay is the best way to handle users' property updates.

[lilleyse]

Looks great, just a couple small comments from me.

[r-veenstra]

I've done some testing, seems to be working as expected. Comments below.

• Clicking the Refresh Tiles button on a tileset or Reload All Tilesets in the Cesium Debug window doesn't result in the WMS overlay updating based on latest values - should it?

• We probably want to include these settings too:
  

[r-veenstra]

@r-veenstra I guess on good max ranges on the zoom level. Can you see if they're reasonable?

The defaults in there look okay and are consistent with Unreal too

[lilleyse]

Clicking the Refresh Tiles button on a tileset or Reload All Tilesets in the Cesium Debug window doesn't result in the WMS overlay updating based on latest values - should it?

Tilesets and raster overlays are now separate things in the C++ layer so OmniTileset would need to explicitly reload the OmniRasterOverlays that it references. @corybarr could you make that fix?

[corybarr]

• We probably want to include these settings too:
  

@r-veenstra This is an issue with all raster overlays. I created #675 for this.

[corybarr]

Clicking the Refresh Tiles button on a tileset or Reload All Tilesets in the Cesium Debug window doesn't result in the WMS overlay updating based on latest values - should it?

Tilesets and raster overlays are now separate things in the C++ layer so OmniTileset would need to explicitly reload the OmniRasterOverlays that it references. @corybarr could you make that fix?

@lilleyse Added.