Enable emptying of command buffer with escape key

This patch is meant to allow the user to cancel a command they've
partially written in the session buffer. For example, if a user in
zathura has mistakenly typed the `g' key instead of `G', they can
cancel their partial command by pressing the Escape key.

girara/callbacks.c

@@ -137,6 +137,18 @@ gboolean girara_callback_view_key_press_event(GtkWidget* widget, GdkEventKey* ev
       session_private->buffer.command = g_string_append_unichar(session_private->buffer.command, codepoint);
     }
 
+    if (session->events.buffer_changed != NULL) {
+      session->events.buffer_changed(session);
+    }
+  } else if (keyval == GDK_KEY_Escape) {
+    if (session_private->buffer.command != NULL) {
+      g_string_free(session_private->buffer.command, TRUE);
+      session_private->buffer.command = NULL;
+    }
+    if (session->global.buffer != NULL) {
+      g_string_free(session->global.buffer, TRUE);
+      session->global.buffer = NULL;
+    }
     if (session->events.buffer_changed != NULL) {
       session->events.buffer_changed(session);
     }