System sharing and art update

Finally I can show you what've been going on in background! I've overhauled the migration system so now it requires you to park at least one ship next to a system for at least two turns before your people will migrate there. The empire influence will need to be reworked, but it's not important right now. The influence map will be used for "radar" and ai processing, and showing your terrority, but that's it.

Systems now can have different planets that's owned by different empires. I've reworked the star name drawing so it will draw different empire colors if the system is occupied by more than one empire.

The first batch of artwork is complete for interface. They are for modular items that can stand by themselves and don't rely on other items, so not everything is replaced yet. The stars are nice, but I miss the white in center, so I will work on that and see if we can work out something. But one thing is for sure, they sure make my placeholder art look really bad :)

Here's the screenshot of a multiple-occupied star system (Sceticrap), and single occupied star (Flous). Note the new fleet icon art, selection art, and other stuff.