Before this a commit a keybind of CTRL + \ would create a GTK
accelerator of <Ctrl>\, which is incorrect. It needs to be
<Ctrl>backslash, which can be retrieved with gdk_keyval_name().
Signed-off-by: Tristan Partin <tristan@partin.io>
Fixes#1802
This allows `keybind` configurations to map to any Unicode codepoint. This enables keybindings for which we don't have a registered keycode or for custom keyboard firmwares that may produce arbitrary text (but the Ghostty support is limited to a single codepoint).
The `keybind` syntax is unchanged. If a bound character doesn't map to a known logical key that Ghostty knows about, we map it to a Unicode codepoint. The unicode codepoint is compared against the _unshifted codepoint_ from the apprt key event.
Note that this binding is to a single _codepoint_. We don't support arbitrary sequences of characters or multi-code point graphemes for keybindings due to the complexity in memory management that would introduce.
This also provides a good fallback for scenarios where it might make sense to educate Ghostty about a key code or fix a bug in our keyboard input system, but the unicode data is correct. In that scenario, unicode key binds should allow key binds to still work while we investigate the input issues.
Example:
```
shift+ö=text:hello
```
This now works as expected on a US hardware keyboard with the Hungarian keyboard layout.
Add additional keypad keys to the encoding scheme. This allows Ghostty
to report KP_HOME and it's relatives. We also always check for a keyval
first, so we can report KP_7, etc as opposed to ASCII '7'.
