input: make wheel events mappable

Un-grabbed wheel events are now passed through the mouse binding
matching logic, instead of being hardcoded to scrolling the terminal
contents.

They are mappable through the BTN_BACK and BTN_FORWARD buttons.

Since they're not actually button *presses*, they never generate a
click count other than 1. This limitation is documented, but not
checked in the config. This means it's possible to create bindings
like "BTN_BACK+3" (i.e. triple "click"). They will however never
trigger.

The old, hardcoded logic is now accessible through the new
scrollback-up-mouse and scrollback-down-mouse mouse
bindings. They (obiously) default to BTN_BACK and BTN_FORWARD,
respectively.

Example usage: keep the default of scrolling terminal contents with
the wheel, when used without modifiers, but map Control+wheel to font
zoom in/out:

  [mouse-bindings]
  font-increase=Control+BTN_FORWARD
  font-decrease=Control+BTN_BACK

(this also keeps the default key bindings to zoom in/out; ctrl-+ and
ctrl+-)

Closes #1077

CHANGELOG.md

@@ -50,6 +50,13 @@
 ### Added
 
 * Support for building with _wayland-protocols_ as a subproject.
+* Mouse wheel scrolls can now be used in `mouse-bindings`
+  ([#1077](1077)).
+* New mouse bindings: `scrollback-up-mouse` and
+  `scrollback-down-mouse`, bound to `BTN_BACK` and `BTN_FORWARD`
+  respectively.
+
+[1077]: https://codeberg.org/dnkl/foot/issues/1077
 
 
 ### Changed