Change mouse buttons to x11 map and libevdev names

This modifies the way mouse bindings are parsed. Instead of adding to
BTN_LEFT, which results in button numbers that may not be expected,
buttons will be parsed in one of the following ways:

1. `button[1-9]` will now map to their x11 equivalents. This is already
the case for bar bindings. This adds support for binding to axis events,
which was not possible in the previous approach.

2. Anything that starts with `BTN_` will be parsed as an event code name
using `libevdev_event_code_from_name`. This allows for any button to be
mapped to instead of limiting usage to the ones near BTN_LEFT. This also
adds a dependency on libevdev, but since libevdev is already a dependency
of libinput, this should be fine. If needed, this option can have dependency
guards added.

Binding changes:
- button1: BTN_LEFT -> BTN_LEFT
- button2: BTN_RIGHT -> BTN_MIDDLE
- button3: BTN_MIDDLE -> BTN_RIGHT
- button4: BTN_SIDE -> SWAY_SCROLL_UP
- button5: BTN_EXTRA -> SWAY_SCROLL_DOWN
- button6: BTN_FORWARD -> SWAY_SCROLL_LEFT
- button7: BTN_BACK -> SWAY_SCROLL_RIGHT
- button8: BTN_TASK -> BTN_SIDE
- button9: BTN_JOYSTICK -> BTN_EXTRA

Since the axis events need to be mapped to an event code, this uses the
following mappings to avoid any conflicts:
- SWAY_SCROLL_UP: KEY_MAX + 1
- SWAY_SCROLL_DOWN: KEY_MAX + 2
- SWAY_SCROLL_LEFT: KEY_MAX + 3
- SWAY_SCROLL_RIGHT: KEY_MAX + 4

include/sway/input/cursor.h

@@ -5,6 +5,11 @@
 
 #define SWAY_CURSOR_PRESSED_BUTTONS_CAP 32
 
+#define SWAY_SCROLL_UP KEY_MAX + 1
+#define SWAY_SCROLL_DOWN KEY_MAX + 2
+#define SWAY_SCROLL_LEFT KEY_MAX + 3
+#define SWAY_SCROLL_RIGHT KEY_MAX + 4
+
 struct sway_cursor {
 	struct sway_seat *seat;
 	struct wlr_cursor *cursor;