osc: add support for osc 176 (app ID)

This adds support for a new OSC escape sequence: OSC 176, that lets
terminal programs tell the terminal the name of the app that is
running. foot then sets the app ID of the toplevel to that ID,
which lets the compositor know which app is running, and typically
sets the appropriate icon, window grouping, ...

See: https://gist.github.com/delthas/d451e2cc1573bb2364839849c7117239

README.md

@@ -499,6 +499,7 @@ with the terminal emulator itself. Foot implements the following OSCs:
 * `OSC 117` - reset highlight background color
 * `OSC 119` - reset highlight foreground color
 * `OSC 133` - [shell integration](#shell-integration)
+* `OSC 176` - set app ID
 * `OSC 555` - flash screen (**foot specific**)
 * `OSC 777` - desktop notification (only the `;notify` sub-command of
   OSC 777 is supported.)

doc/foot-ctlseqs.7.scd

@@ -684,6 +684,11 @@ All _OSC_ sequences begin with *\\E]*, sometimes abbreviated _OSC_.
 |  \\E] 133 ; A \\E\\ 
 :  FinalTerm
 :  Mark start of shell prompt
+|  \\E] 176 ; _app-id_ \\E\\ 
+:  foot
+:  Set app ID. _app-id_ is optional; if assigned,
+   the terminal window App ID will be set to the value.
+   An empty App ID resets the value to the default.
 |  \\E] 555 \\E\\ 
 :  foot
 :  Flash the entire terminal (foot extension)

osc.c

@@ -910,6 +910,10 @@ osc_dispatch(struct terminal *term)
         }
         break;
 
+    case 176:
+        xdg_toplevel_set_app_id(term->window->xdg_toplevel, *string ? string : term->conf->app_id);
+        break;
+
     case 555:
         osc_flash(term);
         break;