Drop external folders onto the Tree View to create new project folders

[winstliu]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions

Description of the Change

Implements the popularly-requested feature #480. Basically if no existing folder is selected the Tree View adds the entries in the data transfer event as new project folders. There are a few rough edges with this implementation, such as but not limited to:

• Folders must be dropped to a blank section of the Tree View after the last entry. This means that if the Tree View scrolls you will be unable to create a project folder as the entries will get added to the last project.
  • I would like to add support to be able to create project folders when dropping them between project folders, similar to how root drag-and-drop works currently.
• Cannot drag and drop folders onto the Tree View when no Tree View exists.

Alternate Designs

No alternatives were considered.

Benefits

External folders can now be dragged-and-dropped into the Tree View to be added as project folders.

Possible Drawbacks

I don't believe there are any drawbacks with this other than the limitations addressed above.

Applicable Issues

Fixes #480

[liuderchi]

Would e.dataTransfer.files be undefined ?

[winstliu]

Good question. files is a standard DataTransfer property that will always be available. https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer/files

[liuderchi]

Thanks for the info. BTW it says that dataTransfer exists on all drag events. Good News.

[winstliu]

I looked into implementing drag-and-drop indicators. It will be harder than I thought because then you'll have to differentiate between moving external folders into existing projects or whether to create a new project folder for it. I'll punt that for a later PR.

[here]

Note that as of 2018-03-06 , this request remains unreleased in the Atom core editor with the tree-view core package at v221.3 or v222 based on https://github.com/atom/tree-view/releases

[liuderchi]

Share a shell snippet for checking releases

# atom latest releases and corresponding tree-view version
$ curl https://api.github.com/repos/atom/atom/releases/latest -s | grep 'tag_name'

  "tag_name": "v1.24.0",

$ curl https://raw.githubusercontent.com/atom/atom/v1.24.0/package.json -s | grep 'tree-view'

    "tree-view": "0.221.3",

[winstliu]

@here we're dealing with tree-view spec failures that need to be fixed before we feel comfortable updating it in Atom itself. Sorry for the delay.