Added mouse button aliases

[vallentin]

This might be a personal preference, but I usually have MouseButton imported, which would make it easier to do MouseButton::Left, compared to importing glfw::MouseButtonLeft.

Additionally, this is also sliiiightly "safer" to use, as if someone forgets to import glfw::MouseButtonLeft, then matching on a MouseButton and having MouseButtonLeft in a match arm, that would only produce a warning. While forgetting to import MouseButton when using MouseButton::Left, would cause an error.

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I can refactor some of the MouseButtonLeft (and friends) doc references, to instead point to MouseButton::Left (and friends), if that is desired?