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.
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
Merge remote-tracking branch 'kee/develop' into develop

Conflicts:
	Source/Eto.Test/Eto.Test/Sections/Controls/GridViewSection.cs
Merge pull request #221 from keepwn/develop

Fix some bugs and add new function BeginEdit