Implemented custom directory structure

[rainman110]

This PR implements a custom directory structure for fbs.
Note: the previous directory structure is still used by default.

Implementation details

A customized directory structure is implemented as follows:

• Default directories are configured in _defaults/src/build/settings/base.json
• If the file fbs_directories.json in the root directory exists
  it will be read by fbs. Here, custom paths can be set, with the following options
  • python_dir : Directory of the python code
  • icons_dir : Directory of the icons
  • resources_dir: Directory of other resources.py
  • settings_dir: Directory of the fbs settings
  • freeze_config_dir: Configuration files for the freeze command
• Eventually, the remaining fbs settings are read
  (either from the default or user configured path)

I had to alter the check for the root directory, as it assumed a fixed
directory structure. Now, it will check for the default settings path
or the existence of the fbs_directories.json file.

Testing

I tested the functionality on the fbs test app, that is created by the
fbs tutorial. I tested the commands run, freeze and installer
successfully. I tested only on windows though. Mac and Linux are
yet untested.

Closes #268

[CLAassistant]

All committers have signed the CLA.

[mherrmann]

Thank you for this PR. I'm a little weary of merging it because it complicates the code quite a bit. In the years since fbs has existed, this is also the first time that this requirement comes up. So I'm hesitant to make changes that (so far) one person has requested, but that will affect me and anybody else who touches fbs's code for the foreseeable future.

I'm leaving this PR open for others, or in case the requirement comes up again. If you'd like to discuss another arrangement where your employer pays me to merge it into fbs, along with an fbs Pro license, let me know. I hope my stance on this is understandable.

[rainman110]

Thanks for looking into the PR.

I can understand your concern about having changes which you are uncomfortable with. The only bigger changes is actually the function load_settings_from_path. It think it would be a good idea for the future, to have a suite of regression tests that enable bigger code changes without having to worry.

I am wondering though, that nobody asked for this feature yet as I think, that a packaging tool should have this flexibility and not dictate a certain file structure, which is also different from most of the python projects out there. I am okay though in having this PR open for future reference.

This PR is solely for private reasons, so there's no company involved here ;)

[ajvogel-motus]

I find it odd that this requirement has only came up once. It was literally the first thing I thought of when looking into fbs! Would definitely add my +1 to this being merged.

[gabrieljreed]

Also adding my +1 to this feature being added, I was equally surprised to find that nobody else has requested this feature. Thanks for your work on this PR Martin!