input: modify interpretation of doubleclick events
The code combining button presses into multibutton commands prevented
single click commands from triggering if a doubleclick event had been
generated from the same button press. As a result using the mouse
wheel to seek worked very badly. Special-case doubleclick events in
the event interpretation code to avoid this issue. This changes the
sequence of generated "keys" for press-release-press-release from
MOUSE_BTN0 MOUSE_BTN0-MOUSE_BTN0_DBL MOUSE_BTN0_DBL to
MOUSE_BTN0 MOUSE_BTN0_DBL MOUSE_BTN0.
"Keys" like MOUSE_BTN0-MOUSE_BTN0_DBL will never be generated now; any
existing configuration files using those need to be changed.