Merge pull request #296 from tradej/master

Window with info about installing new Assistants
Rename modify to tweak and task to extras
Remove the submodule => finally no default assistant set
Fix some docs after removing default assistant set and renaming assistants
Reverse DATA_DIRECTORIES order to prefer user-installed assistants from DAPI over systemwide assistants
Fixed da_auth to accept Heredoc and not arguments (fixes #297)
Remove forgotten debugging print
Reflect DAPI and other changes in README
Some readme tweaks and fixes
Enforce argument attributes' names (fixes #298)
Adapt SYSTEM_DEPTYPES_SHORTCUTS to recent changes in get_distro_name()
Fix gui tests after assistant categories renaming
Nicer output when no package manager works