Rework fomod installer backend

Changed:
  - User's choices are now "saved" when going to previous pages
  - Folders to install are now properly merged based on priority
  - Omitted destination attributes in file/folder tags are now
    properly handled.

TODO:
  - Update tests in test_bash repo
  - Add some inline documentation

This must go into the final commit message:

Ignored tags:
  - moduleImage
  - fommDependency

Ignored attributes:
  - position, colour - [moduleName]
  - alwaysInstall, installIfUsable - [file, folder]

Author: Daniel Nunes