[feature request] share folder content (recursive) instead of folder-name

[k3b]

In MaterialFiles you can select one or more files and share them to other apps (i.e. send via bluetooth or via email)

When "the share" contains a directory-entry the share-consuming-app only gets the folder alias of the used FileProvider which cannot be processed further because FileProvider does not support browsing folders (see comment in https://stackoverflow.com/questions/48062097/how-to-open-a-directoryfolder-in-android-using-file-provider ).

As a result the share-consuming-app may crash (see MaterialFiles#575 or ToGoZip#28)

Instead of sharing the selected folder name share all files that are inside the folder (recursivly)

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Example: (current working)

In MaterialFiles select the folder "sd_sub_test" and press "share"
MaterialFiles shares "sd_sub_test" with mimetype="vnd.android.document/directory"

requested change:

MaterialFiles shares all files inside the folder "sd_sub_test" (i.e. "sd_sub_test/test.txt", "sd_sub_test/Image.png") instead of sharing the folder name