Add WithRangeCircle trait for drawing arbitrary ranges.
Add proximity trigger plumbing.
Add UpgradeActorsNear trait for passive upgrades.
Add the stealth generator to TS.
Extend INotifyOtherProduction to all actors.
Apply upgrades to units produced within range immediately.
Fix spy tooltip definition.  Fixes #6665.
Revert "build with latest Mono from @Xamarin"

This reverts commit f1bfbf933bb9cc170926233294f9ef9704cd3e7e.
Merge pull request #6672 from pchote/revert-mono

Revert "build with latest Mono from @Xamarin"
Merge pull request #6666 from pchote/fix-spy-tooltip

Fix spy tooltip definition.
Merge pull request #6644 from pchote/proximity-triggers

Closes #2175
Use the existing SpriteLoader cache for tilesets.
Use SpriteLoader for frame exporting.
Remove --transpose utility command.

The engine has supported transposed sprites for a long time now.
Add plumbing for mod-defined sprite loaders.
Move ShpTD sprite loading into Mods.Common.
Move ShpTS sprite loading into Mods.Common.
Move R8 sprite loading into Mods.D2k.
Move TmpRA sprite loading into Mods.Common.
Move TmpTD sprite loading into Mods.Common.