Several more improvements importing FB projects

- Parsing of directory variables.
- Better detection of the external themes path.
- Support for custom themes with non-standard names.
- Support for SDK themes.
- Support for packaged sourced paths.
- Support for packaged libraries.
- Force the current working directory to the imported project directory
to solve problems with relative paths.
Merge pull request #453 from Neverbirth/airimprovements

Several more improvements importing FB projects