Implement Editable ComboBox.
- implement editable comboBox in all platform.
- in winrt, the editable property always be false.
- in mac, the selectionDidChange event may be still a problem.
Merge remote-tracking branch 'kee/develop' into develop

Conflicts:
	Source/Eto.Mac/Forms/Controls/ComboBoxHandler.cs
	Source/Eto.WinRT/Forms/Controls/ComboBoxHandler.cs
	Source/Eto.Wpf/Forms/Controls/ComboBoxHandler.cs
remove ComboBoxHandler.cs.BASE.cs,ComboBoxHandler.cs.LOCAL.cs,ComboBoxHandler.cs.REMOTE.cs
set AutoInitialize to false in ComboBox.IHandler interface
Change `ComboBox` to `DropDown`, and add new `ComboBox`.
`DropDown` is a drop down control, and `ComboBox` is a subclass of `DropDown`, which be added the ability to edit a text filed.
organize projects
Change `ComboBox` to `DropDown`, and add new `ComboBox`.
`DropDown` is a drop down control, and `ComboBox` is a subclass of `DropDown`, which be added the ability to edit a text filed.
Merge branch 'develop' of github.com:keepwn/Eto into develop
Merge pull request #232 from keepwn/develop

Implement Editable ComboBox.