input: update serial on each key press/release and mouse button

The serial is used when copying/pasting data from the clipboard. Up
until now, we’ve used the serial from the keyboard/mouse enter
events.

This works in most cases, but breaks in the following example:

  $ wl-copy  WLCOPY
  /* Ctrl+Shift+v works fine (pastes "WLCOPY") */

  $ printf "\033]52;c;eHl6\a"
  /* Ctrl+Shift+v pastes "WLCOPY" instead of "xyz" */

Shifting focus away and then back to the foot window, and re-executing
the printf works, suggesting the “enter” serial is no longer valid
after another process(?) has copied something to the clipboard.

Updating the serial on key press/release (and the corresponding mouse
serial on mouse button events) seems to fix this.

I’ve also tested that “normal” copy/paste operations, within the same
foot instance, and between foot and other applications, are still
working. In at least river (wlroots based), and GNOME/mutter.

Closes #753

CHANGELOG.md

@@ -61,6 +61,8 @@
 * Fix crashes after enabling CSD at runtime when `csd.size` is 0.
 * Convert `\r` to `\n` when reading clipboard data
   (https://codeberg.org/dnkl/foot/issues/752).
+* Clipboard occasionally ceasing to work, until window has been
+  re-focused (https://codeberg.org/dnkl/foot/issues/753).
 
 
 ### Security