Add a keymatrix_t type

This contains both the matrix number and key position, in preparation
for multi-matrix support

tmk_core/common/keyboard.c

@@ -238,7 +238,8 @@ void keyboard_task(void)
                 for (uint8_t c = 0; c < MATRIX_COLS; c++) {
                     if (matrix_change & ((matrix_row_t)1<<c)) {
                         action_exec((keyevent_t){
-                            .key = (keypos_t){ .row = r, .col = c },
+                            // The main matrix is always 0
+                            .key = (keymatrix_t){.pos = (keypos_t){ .row = r, .col = c }, .matrix = 0},
                             .pressed = (matrix_row & ((matrix_row_t)1<<c)),
                             .time = (timer_read() | 1) /* time should not be 0 */
                         });