Create download management & notification using GeckoView

[Daddoon]

On the current download light implementation present in the GeckoView Xamarin.Forms project, content is redirected to the system DownloadManager.

As for evolution and for more complex scenario, like downloading with a cookie present in the current session, the download scenario should:

• Use GeckoWebExecutor Streaming API in order to download data
• Notify current download to user using a system notification on the statut bar. - See this Stackoverflow post
• Being able to add the downloaded file to the DownloadManager history. - See Android documentation
• Open the downloaded file with compatible Intent when download is finished - We can use the same code implementation present on the current DownloadManager implementation.