Update <TitlePortal> to allow customization of the the page title style

[fzaninotto]

Problem

The <TitlePortal> component fixes the page title style without possibility to customize it.

Solution

Accept extra props

export const MyAppBar = () => (
    <AppBar>
        <TitlePortal variant="body2" component="h3" />
        <SettingsButton />
    </AppBar>
);

Closes #8871