Add chrome for slider thumb.

update hardcoded references to chrome.yaml to use public parameters.
Merge pull request #6814 from steelphase-forks/bleed-slider-thumb

Add chrome for slider thumb.
Load tmp(ts) ExtraData for cliffs etc.
WIP: Add Tileset support for per-tile minimap color and height.

TODO: Fix PickAny support.
Convert TS terrain height, ramp, and minimap colors.
Introduce TerrainTemplatePreviewWidget.
WIP: Hook up basic city buildings.

TODO Footprints.
Add demo map used for news screenshots.
Introduce ActorPreviewWidget and associated plumbing.
WIP In-game map editor.