urls: add key binding that toggles whether URLs are displayed on jump-label

By default, the URL isn’t shown on the jump-label. For auto-detect
URLs, doing so is virtually always useless, as the URL is already
visible in the grid.

For OSC-8 URLs however, the URL is often _not_ visible in the
grid. Many times, seeing the URL is still not needed (if you’re doing
‘ls --hyperlink’, you already know what the URIs are).

But it is still useful to have a way to show the URLs.

This patch adds a new key binding action that can be used in url-mode
to toggle the URL on and off in the jump label.

It is bound to ctrl+t by default.

foot.ini

@@ -125,6 +125,7 @@
 
 [url-bindings]
 # cancel=Control+g Control+d Escape
+# toggle-url-visible=Control+t
 
 [mouse-bindings]
 # primary-paste=BTN_MIDDLE