ui: Add insert rule below button

This change adds an explicit button to add a new rule. Previously a new
rule could only be created via context menu, which is hard is not
beginner-friendly and not obvious. This change improves that.

Related pwr-Solaar#2349

lib/solaar/ui/diversion_rules.py

@@ -203,6 +203,12 @@ def _create_button_box(self):
         button_box = Gtk.HBox(spacing=20)
         button_box.set_margin_start(10)
         button_box.set_margin_end(10)
+        self.insert_rule_btn = Gtk.Button.new_from_icon_name("document-revert", Gtk.IconSize.BUTTON)
+        self.insert_rule_btn.set_label(_("Insert new rule"))
+        self.insert_rule_btn.set_sensitive(False)
+        self.insert_rule_btn.set_valign(Gtk.Align.CENTER)
+        self.insert_rule_btn.connect("clicked", lambda *_args: ())  # TODO insert rule below
+        self.save_btn = Gtk.Button.new_from_icon_name("document-save", Gtk.IconSize.BUTTON)
         self.save_btn = Gtk.Button.new_from_icon_name("document-save", Gtk.IconSize.BUTTON)
         self.save_btn.set_label(_("Save changes"))
         self.save_btn.set_always_show_image(True)
@@ -215,6 +221,7 @@ def _create_button_box(self):
         self.discard_btn.set_valign(Gtk.Align.CENTER)
         self.save_btn.connect("clicked", lambda *_args: self._save_yaml_file())
         self.discard_btn.connect("clicked", lambda *_args: self._reload_yaml_file())
+        button_box.pack_start(self.insert_rule_btn, False, False, 0)
         button_box.pack_start(self.discard_btn, False, False, 0)
         button_box.pack_start(self.save_btn, False, False, 0)
         button_box.set_halign(Gtk.Align.END)