Ensure that the DirectoryService's Chooser uses a valid directory path

[Ayowel]

Issue

Fix #514

If the working directory saved by AsciidocFX is not a path to a valid directory or subdirectory, changing the working directory becomes impossible from the GUI (clicking the button to set the working directory fails to open a prompt).

Fix

With this PR, the first valid parent directory found is used to browse and if none exists we use the first drive found.

Test

All tests were performed on windows, all commands executed in cmd

Test path bubble-up

• Create a custom workdir under several subdirectories : mkdir my/custom/workdir/for/tests
• Start AsciidocFX and set the workdir to my/custom/workdir/for/tests
• Delete the whole directory tree rm -r my (real use-case requires to close AsciidocFX first and restart it after)
• Try to change the working directory by pressing the icon

Test Drive failover

• Create a custom workdir drive subst K: %cd%
• Start AsciidocFX and set the workdir to K:\
• Delete the whole directory tree subst /D K: (real use-case requires to close AsciidocFX first and restart it after)
• Ttry to change the working directory by pressing the icon

[rahmanusta]

Hi @Ayowel, thanks for the implementation