Remove feature descriptions in FEATURES.md

[madsmtm]

Builds upon #3478. Related to #3431.

I went through all of the text in FEATURES.md, and cross-referenced it with our code, and as I suspected, the information in the code is much more detailed and up to date. So I propose we remove most of the contents in FEATURES.md, as it is difficult to keep up to date, and I don't really see the value provided.

Only thing I'm unsure about is the following three points that are yet unimplemented, and as such doesn't have a clear reference in our code:

• Popup / modal windows: Windows can be created relative to the client area of other windows, and parent windows can be disabled in favor of popup windows. This feature also guarantees that popup windows get drawn above their owner.
• Gamepad/Joystick events: Capturing input from gamepads and joysticks.
• Device movement events: Capturing input from the device gyroscope and accelerometer.

[daxpedda]

I think this is the right way to go.
Will take a closer look when its ready.

[kchibisov]

bump

[daxpedda]

LGTM.

My small nit can be addressed in a follow-up.

[daxpedda]

I feel like this file is now quite small and its contents should either live in README.md, CONTRIBUTING.md or be removed entirely.