Support the HTMLInputElement:webkitdirectory API

[Jaifroid]

Fixes #421.

[Jaifroid]

Tested and working well on Firefox and Edge Legacy.

Doesn't interfere with IE11 legacy file picking, and doesn't prevent use of newer APIs if available.

Need to check interaction with Electron and NWJS.

TODO: put repeated code into a function.

[Jaifroid]

It seems the API has poor support on Android even though webkitdirectory is reported as available.

[Jaifroid]

No support on any mobile browser: https://caniuse.com/input-file-directory.

[Jaifroid]

Prevented false-positive detection on iOS and Android, and tested on BrowserStack.

[Jaifroid]

I've now made a single function to process a directory of files, whether it is provided by webkitdirectory or by File System Access API.

It would be nice if the list of files were retained and displayed on reload of the app. Assuming this is possible (it's just a cookie list), then clidking on a file in the list should open the directory picker in Firefox. This would put us at near parity between the two APIs.

[Jaifroid]

Last thing to do is make it so refresh button opens directory dialogue.

[Jaifroid]

Checks:

• IE11
• Edge Legacy
• UWP (gives message about missing dragged archive twice - once on load, once when picking a folder, but it's minor)
• Firefox 45 - as expected, only in jQuery mode, and user has to switch to it manually (see README.md)
• Firefox 60
• Chrome 49 on Windows XP
• Chrome 50
• Modern Firefox
• Modern Chromium (Edge)
• Electron on Windows
• Electron on Linux
• PWA on iPad
• PWA on iPhone
• PWA on Android
• NWJS on modern Windows
• NWJS for Windows XP

[Jaifroid]

All manual test passed. Ready to merge. One minor issue noted above has been noted in #427.