dir-picker: add contextmenu "open in filemanager"

[k3b]

In the directory picker there should be a context-menu "Open in filemanager" that opens any installed filemanager to show all files/subdirectories that are inside the selected folder.

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Technical details:

• action=android.intent.action.VIEW
• data=file:///path/to/directory/
• mime=vnd.android.document/directory (android api-19 DocumentsContract.Document.MIME_TYPE_DIR)

alternatives used

• data=some content provider specific directory info
• mime=resource/folder (used by some filemanagers see https://stackoverflow.com/questions/17165972/android-how-to-open-a-specific-folder-via-intent-and-show-its-content-in-a-file ) or

see also syncthing/syncthing-android#838

[k3b]

implemened in 0.6.4