osc: kitty notifications: buttons, icons, app-name, categories etc

First, icons have been finalized in the specification. There were only
three things we needed to adjust:

* symbolic names are base64 encoded
* there are a couple of OSC-99 defined symbolic names that need to be
  translated to the corresponding XDG icon name.
* allow in-band icons without a cache ID (that is, allow applications
  to use p=icon without having to cache the icon first).

Second, add support for the following new additions to the protocol:

* 'f': custom app-name, overrides the terminal's app-id
* 't': categories
* 'p=alive': lets applications poll for currently active notifications
* 'id' is now 'unset' by default, rather than "0"
* 'w': expire time (i.e. notification timeout)
* "buttons": aka actions. This lets applications add additional (to
  the terminal defined "default" action) actions. The 'activated' event
  has been updated to report which button/action was used to activate
  the notification.

To support button/actions, desktop-notifications.command had to be
reworked a bit.

There's now a new config option:
desktop-notifications.command-action-arg. It has two template
arguments ${action-name} and ${action-label}.

command-action-arg gets expanded for *each* action.

${action-name} and ${action-label} has been replaced by ${action-arg}
in command. This is a somewhat special template, in that it gets
replaced by *all* instances of the expanded actions.

terminal.c

@@ -3601,7 +3601,9 @@ term_bell(struct terminal *term)
     if (term->conf->bell.notify) {
         notify_notify(term, &(struct notification){
             .title = (char *)"Bell",
-            .body = (char *)"Bell in terminal"});
+            .body = (char *)"Bell in terminal",
+            .expire_time = -1,
+        });
     }
 
     if (term->conf->bell.flash)