Update 0.1.8

First implementation of a bookmark-system.

You can now save your favourite URLs to a dropdown list below the adress bar.

Important notes:
- the app needs permission to write data on your device now! (make sure to check permission settings)

- a .json file containing your url strings is saved under /Android/data/com.AutosyphonStudios.XRBrowserForPico4/files

- make sure to make backups of this file if you don't want to loose it, you can also edit it externally if you need.

- in this version (0.1.8) the saved links will only be loaded in the center webview (but you can send the link from there to the left and right webviews)

- the dropdown list needs to contain at least 2 URLs to work properly (classic Unity troubles...)