Documentation and small fixes

Changes:

• There is now documentation!
  Reasonable improvements in documentation process meant that I only had to type out about 6K symbols / <700 words total.
• I have fixed a curious issue with cursed-looking Alt-Space menu, because apparently that's what you get if you try to show a menu on a child window. The menu is now forbidden from showing up.
• I have finally fixed an issue with gamepad discovery/loss events not triggering after focusing away from the window and back.
  Turns out that there was a separate WinAPI event that should have been forwarded, but was not.
• I have fixed a buffer error with window_frame_get_rect function.
  No one actually complained about it, supposedly because no one ever used it.