Multi-.Oni: No conflict, priority based resolution

BSL:
1. Mods containing different BSL-files for the same level (addon=no):
"light" warning window

2. Mods containing (partially) same BSL-files for the same level (addon=no):
Conflict warning, if accepted take only files of higher priority mod

3. Mods containing (partially) same BSL-files for the same level (addon=yes):
no conflict, overwrite base files with addon-files

4. As 3 with Addon on Addon:
("Light" warning?) conflicting files are resolved by higher priority addon taking precedence