UWP: Add disable autofocus support

Disable autofocus support for iOS/Android

Merge pull request Redth#533 from Redth/feature

Fix Forms default overlay bindings

Nuspec points to release notes url

Refactored MobileBarcodeScanner, ScanPage and ZXingScannerControl

MobileBarcodeScanner no longer sets static properties, but instead passes instances of config/settings via the page navigation.  This greatly reduces clutter in the ScanPage
ScanPage is also no longer responsible for trying to handle back navigation, and MobileBarcodeScanner will take care of this.  This means ScanPage could in theory be used independently of MobileBarcodeScanner now (it also now implements IMobileBarcodeScanner to follow the same pattern that iOS/Android do with pages/views).

Forms: Target Profile 259 to match the non-forms library

Remove -beta from build artifacts

Added try/catch around autofocus, hopefully fixes Redth#242

try to get appveyor build version from environment variable