[6.x] Form Builder sidebars

[duncanmcclean]

This pull request reworks how the left/right sidebars in the Form Builder work, allowing us to keep all the logic contained within the layout and the page component.

Pages can use the new <LayoutPanel> component to inject content into the relevant panel/sidebar:

<LayoutPanel side="left">
    <p>Fieldtypes</p>
</LayoutPanel>

<p>Fields</p>

<LayoutPanel side="right">
    <p>Settings</p>
</LayoutPanel>

Replaces #14369