w32_common: capture mouse input

Mouse buttons can get stuck down if the button is pressed inside the
video window and released outside. Avoid this by capturing mouse input
when a button is pressed.
w32_common: prevent MOUSE_BTN0 sticking after drag

The window doesn't recieve a WM_LBUTTONUP message after it's dragged,
probably because it's swallowed by the modal loop. To stop the button
from sticking, release it manually when the drag is complete.
DOCS: add coding style

I've tried not to be too detailed (because it's not a reference, just
some guidelines), but it still got relatively long. Also contains
conventions for sending patches.
DOCS/tech-overview: DOCS/OUTDATED-tech/ was deleted