do not allow access to preferences when a transaction is running

src/manager_window.vala

@@ -51,6 +51,8 @@ namespace Pamac {
 		[GtkChild]
 		Gtk.Button button_back;
 		[GtkChild]
+		Gtk.ModelButton preferences_button;
+		[GtkChild]
 		Gtk.TreeView packages_treeview;
 		[GtkChild]
 		Gtk.TreeViewColumn packages_state_column;
@@ -1428,6 +1430,11 @@ namespace Pamac {
 			refresh_packages_list ();
 		}
 
+		[GtkCallback]
+		void on_menu_button_toggled () {
+			preferences_button.visible = !transaction_running;
+		}
+
 		[GtkCallback]
 		void on_history_button_clicked () {
 			transaction.run_history_dialog ();

src/updater_window.vala

@@ -25,6 +25,8 @@ namespace Pamac {
 		[GtkChild]
 		Gtk.HeaderBar headerbar;
 		[GtkChild]
+		Gtk.ModelButton preferences_button;
+		[GtkChild]
 		Gtk.StackSwitcher stackswitcher;
 		[GtkChild]
 		Gtk.ScrolledWindow repos_scrolledwindow;
@@ -148,6 +150,11 @@ namespace Pamac {
 			}
 		}
 
+		[GtkCallback]
+		void on_menu_button_toggled () {
+			preferences_button.visible = !transaction_running;
+		}
+
 		[GtkCallback]
 		void on_preferences_button_clicked () {
 			transaction.run_preferences_dialog.begin (() => {