Define key name and state constants

The header 'Include\ri.h' defines several names  to _simplify condition
testing_ and reading:

 * `RI_MOUSE_X{ 1, 2 }_BUTTON_{ DOWN, UP }` becomes a synonym for mouse
   buttons 4 and 5 being either up or down (key 4 is the first extension
   key, and 5 is the second); and
 * `RI_KEY_STATE_{ DEPRESSED, RELEASED, UNCHANGED }` are group of three
   macros that indicates the direction of a key (`DEPRESSED` is a key
   being pressed down, `RELEASED` is a key being let go, and `UNCHANGED`
   is the statement that the key has not changed state).

The raw input API presents _descriptive_ names to the first 3 buttons
found on the mouse (the left, right, and middle buttons) but then stops
short for the extension buttons. However numbered names for the first
three (`RI_MOUSE_BUTTON_{ 1, 2, 3 }_{ DOWN, UP }`) but not names
descriptive names for the extension keys. Rather than become confused
about which key is which number the addition of these names completes
both naming conventions.