Merge pull request #585

* Enable building for Windows on other operating systems

* Copy DLLs build by plugin projects to the plugins folder using a Copy…

* Copy plugins using Copy tasks for WinForms and Tests projects

#580 - Fix fleets being deleted when seen by other empires (#581)

#570 - fix subwindows appearing behind the main window when invoked f…

#571  - Fix crash when processing a battle (#572)

Battles were crashing when copying seekers.

#573 - Fix crash when ships get downgraded to an old design after tra…

#557 - Fix facility upgrades overspending available resources (#566)

#542 - Restore missing log icons (#568)

Allow creating references to objects that don't exist in the current …

#525 - Restore missing goto buttons on the turn log (#569)

#565 - Fix ships' supply pools being shared (#567)

Among ships that have the same number of supplies, instead of each ship
having its own supply pool