enhanced bell configuration

Add a separate section for bell configuration, with a bell-specific
command option and a setting to allow that command to run without regard
to keyboard focus (for those of us who enjoy being beeped at at all
times, for example). The actions are also no longer mutually exclusive;
this is primarily anticipating urgency support which cannot be
replicated outside the process (in server mode anyway) and would thus be
complementary to any notification or arbitrary command.

foot.ini

@@ -32,6 +32,12 @@
 # workers=<number of logical CPUs>
 # osc8-underline=url-mode
 
+[bell]
+# urgent=no
+# notify=no
+# command=
+# command_focused=no
+
 [scrollback]
 # lines=1000
 # multiplier=3.0