Add new sokol_poll.h layer

[Toctave]

Basic polling layer on top of sokol_app's event API. Only supports keyboard input for now.

[floooh]

It's a good PR for discussion, but I'll probably won't merge it, sorry :)

I have a couple more things in mind apart from just managing a key-pressed bitfield:

• have an "any" and "all" function which take bitmasks to check if any of a key group or all of a key group is currently pressed
• then there's the whole problem area of missing presses when the down- and up-event happen in the same frame, so "is key X currently pressed" becomes a "was key X pressed at any time during the last frame" => which in turn means that the library needs to know when a new frame starts... for a solution there should probably a separate bitmask for key-down events and key-up events in a frame, which are then 'consolidated' into a key-pressed bitmask.
• and the 'key mask' should probably be a 'button mask' which at least includes mouse buttons
• and it should also be possible to poll the current mouse position
• ...and while at it, it would be nice to tackle gamepad support, but not sure if this should go into a separate header - but the input polling header should at least somehow integrate the gamepad input.

...etc... etc...

[Toctave]

Yeah I thought of this as a starting point / discussion PR, ofc it won't be merged until it fits what you had in mind.