Allow frigates to trade - #1989

Not pushing this to `release` until we iron out #1999 at the very least.
Fixes #1999 - Don't crash on special unloading and loading combinations
More friendly selection color

This should go better with our new tileset too.
(cherry picked from commit e3c9fe6b3fe40af8d7d2a93e97577deda6842d0c)
Less obtrusive buildability highlights
(cherry picked from commit bf12586c4b08ca6aba51cd01937c8a6c5f55aa5a)
More obtrusive highlights for related buildings

I personally like this better. Revert if you don't :-)
(cherry picked from commit 47c495a5bd7231a112e0e5be7592466d26b0b356)
Require buildings other than warehouses to be entirely within settlements. Fixes #1970. Fixes #969.
Merge branch 'release'