Add support for :keypress property to eventbox

[AlexandrePicavet]

Description

I wanted to be able to close my window by pressing the Escape key, but I couldn't figure out how to do that, so I added a way to handle keypress events on the eventbox widget to execute a command.

Usage

The keypress property is structured in the following way:

Enter |> echo 'You pressed the Enter key'

Enter : The key to press
|> : The separator
echo 'You pressed the Enter key' : The command

Here's an example of how to use it:

(defwindow my-window
	:monitor 0
	:stacking "fg"
	:exclusive false
	:focusable true
	:geometry (geometry
		:anchor "top left"
		:x 0
		:y 0
		:width "100%"
		:height "100%"
	)
	(eventbox
		:keypress "Escape |> eww close my-window"
		(label :text "Press the Escape key to close the window")
	)
)

Showcase

eww-keypress.mp4

Additional Notes

• I wasn't sure which syntax would be best for this feature, so if you have a different suggestion, I can change that.
• It is my first rust contribution, so I may have done some things not quite up to the standards, if so, I'll happily fix them.
• As shown on the video above, I added a debug log on keypress indicating which key has been pressed and which widget caught this event. Do you find this useful, or would you prefer that I remove it ?

Checklist

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing.