iOS: Add Grid.BeginEdit to fix build
XamMac2: Fix odd error when creating system fonts
Update nuspec and project files to allow packaging using mono and xbuild.
- Include shared Common.targets and Build.targets to customize the build for all projects
- Includes Eto.Build.targets from same level as root folder (if exists), similar to Eto.Common.targets for user-supplied targets
- Update WinRT to VS2013/Windows 8.1
- Nuget packages now require exact version of other eto dependencies to avoid any conflict of versions
Add helpers for each platform to get the native control with a control.ToNative() extension method.