Fix some bugs and add new function BeginEdit

1. fix TextArea's BackgroundColor and TextColor in gtk3.
2. fix TextArea's BackgroundColor in mac.
2. add new function BeginEdit in Grid, all of winform, wpf, gtk2/gtk3, mac/mac64.
Add LinkButton.DisabledTextColor
Mac: LinkButton now is based on a label to allow for wrapping
Mac: When LinkButton is disabled, use disabled color and don't use pointer cursor
Wpf: Enable wrapping for LinkButton and add disabled text color
WinForms: Don't autosize form when client size is set
WinForms: Fix setting Scrollable.Padding
Wpf: Fix TableLayout when set as a child of a control and is not yet loaded
Wpf: Minimize WpfControl's footprint by making the Font a property in the Widget.Properties dictionary as it is not usually used.
Debug exceptions when loading a platform
Add submenu to context menu tests
Update assembly version to v1.99 to prepare for v2.0 release
Gtk: Add synchronization context (if not set) to support async and things like BackgroundWorker.
Fix some in TextAreaHandler

1. [Gtk/Mac]fix could not get TextAreaHandler's BackgroundColor
2. [Mac]when `TextArea's Enable` property status is changing, change BackgroundColor and TextColor too
No longer need to use Application.Initialize event

Also, use ClientSize instead of Size to show best practices setting a form's size.
Update nuspec for xamarin.mac v2, now that it passes the correct platform identifier.
Register overridden OnClick methods when subclassing LinkButton
Update build to attempt to allow mono to package nupkg, still not there yet (can't clean and build without errors).
Add ability to patch AssemblyInformationalVersion of GlobalAssemblyInfo.cs when building
Update breaking changes
Add Mac64 build of the test app
WinForms: Fix issues with initial control positioning within scrollable controls, and make sure layouts are resumed before being shown.