[Mac] Optimized MacOSKeyCode translation

This also reduces memory allocations on startup by ~10K.
[SDL2] Optimized Sdl2KeyCode translation

Also fixed Key.WinLeft, WinRight and Menu translation.
[Examples] Improved ExternalContext test

Also added documentation
[OpenTK] Fixed DllImport library for OpenGL on Mac OS X
[Bind] Do not generate slots for OpenGL <= v1.1

We can use DllImports for OpenGL functions <= v1.1 on all platforms,
including Windows. This allows us to reduce the number of required
GetProcAddress calls by 474, reducing startup time and memory
consumption.

This setting can be turned off through Settings.Legacy.UseDllImports
and UseWindowsCompatibleGL.
[OpenTK] Rebased on develop
[Win] Replaced KeyMap dictionary with switch

This reduces the amount of allocations on the startup path.
[Input] Use Key.LastKey instead of Enum.GetValues

This avoids unnecessary memory allocations in the KeyboardDevice
constructor.
[ES] ES10 does not require initialization

It consists solely of DllImports that will be initialized on first use,
regardless of the existence of an OpenGL context.
[Win] Wgl no longer inherits GraphicsBindingsBase

This is a necessary step for the next commit.
[Bind][ES][GL] Output entry points as byte[]

Moving from string[] to byte[] reduces startup memory allocations
tremendously (up to 70% lower memory use on Windows!)
[X11] Optimized XKey translation
[Graphics] Redirect GetAddress(string)

GetAddress(string) now calls GetAddress(IntPtr) internally. This
reduces the number of APIs that must be implemented for each platform.
[Graphics] GetAddress(string) is no longer needed
[Mac] Sped up extension loading

From ~200ms down to ~65ms on a rMBP with Nvidia 650M and Mac OS X
10.9.2.
[Bind] Do not generate DllImports

These break dynamic loading of OpenGL ES and bloat OpenTK.dll with
duplicated strings. The new extension loading mechanism is now fast
enough to make DllImports unnecessary.
[GL][ES] Do not break public API

Even if that API is not meant to be used by applications.
[GL][ES] Regenerated bindings
[Win] Fixed warnings

Protected object in static class and wrong parameter names in
documentation
[X11] Implemented new-style bindings for GLX

Replaced delegates with call instructions and completely removed the
old extension loading code.