Alternate assets folder

**What do you want to see?**
The ability to specify an "alternate assets folder" for FrEee which the game will fall back on if assets such as graphics are not found.

**How would it help players?**
Players could then link FrEee to their SE4 folders, without having to actually copy all the assets over (the pictures can be multiple gigabytes if you have a lot of shipsets because SE4 uses uncompressed bitmaps).

**Alternatives**
An importer to copy the assets maybe?

**Screenshots and mockups**
N/A

**Things to consider**
N/A