Everything related to file system access has been rewritten to make use
of the storage access framework. This means that the
WRITE_EXTERNAL_STORAGE is no longer needed. Because of that, the
dependency on permission_handler could be dropped and all code related
to permission handling has been removed. To be able to open a whole
document tree, the minSdkVersion was bumped to 21. Finally the file
selector was rewritten using custom platform dependent code.
The role and instrument tables have been merged into one (the instrument
table). There are not that many roles that aren't instruments and it is
much simpler to mange this way. The role editor and role selector have
been removed and the instrument related UI parts have been modified
accordingly.
This required to add a platform channel with a method to get the
toplevel storage devices. The app will need to request the
WRITE_EXTERNAL_STORAGE permission later. Also the dependency on "path"
was introduced.
