Improve documentation for header and toolbar: * Simplify Toolbars demo in component showcase using Tabs. * Simplify Tabs demo in component showcase to match the new documentation * Update MainView documentation * Update Page documentation * Update PageStack documentation * Mark Tab.iconSource as deprecated in documentation (new tabs don't have icons) * Update Tabs documentation * Document ToolbarActions. Fixes: https://bugs.launchpad.net/bugs/1122856, https://bugs.launchpad.net/bugs/1124266, https://bugs.launchpad.net/bugs/1161399.