Added ToolkitOptions class

ToolkitOptions allows the user to influence the implementation of
OpenTK.Platform at runtime.
Refactored environment config detection

This significantly cleans up the startup sequence on all platforms:
- X11 is not detected on non-Linux platforms unless the user explicitly
requests it
- Supports selection of platform abstractions (SDL) vs native
implementations.
- Returns correct flags on Android and iOS.

This contains a semantic change: OpenTK.Configuration will not return
correct values until OpenTK.Toolkit.Init() has been called, either
directly or indirectly (e.g. by creating a window.)
Cleaner startup sequence

OpenTK.Toolkit will now initialize OpenTK.Configuration and
OpenTK.Platform.Factory explicitly. It can also receive an optional
ToolkitOptions parameter to influence the OpenTK.Platform implementation
that will be chosen. Finally, it explicitly implements IDisposable to
clean up after itself.
GLControl requires native backend

SDL does not currently support embedding into Windows Forms (this is an
upstream limitation.) To ensure that existing WinForms applications
continue to function even if SDL is installed, GLControl will now try to
initialize OpenTK with a native backend. The user can still override
this behavior using OpenTK.Toolkit.Init(ToolkitOptions), as normal.
Hack for GLControl support

Added a hack to allow GLControl examples to run even if SDL is
installed.