strafehud: adjust strafehud to match actual behaviour of physics code in various situations
fixed various bugs causing inaccuracies when air strafe turning
take friction into account while on ground
fixed swimming detection and remove code that tries to handle water strafing since swimming uses 3D velocity physics which is not supported by the strafehud
removed hud_panel_strafehud_antiflicker_speed since it is no longer used
tweaked slick detector a bit