Right click menus

**What do you want to see?**
Select a ship, right click a planet, click "colonize" - that sort of thing.

**How would it help players?**
Make the UI more intuitive.

**Alternatives**
Right click a ship, click "colonize", click the planet, a la Cosmic Supremacy. This would be more "discoverable" (easier to learn what the commands are) but the menus would get cluttered.
I wonder if we could make the menu style an option? We'd probably want to programmatically populate the menus on the fly so with the right data model we could probably support both styles (right click ship vs. right click target)...

**Screenshots and mockups**
N/A

**Things to consider**
How to handle commands that don't take a target, e.g. scrapping a ship?
How to handle when there are multiple potential targets in the same sector?