Add modal prop to Sidebar

[keul]

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[ ] bug report
[x] feature request
[ ] support request => Please do not submit support request here, instead see https://forum.primefaces.org/viewforum.php?f=57

Current behavior

Loosely related to #377.

Although is now possible to freely click outside the Sidebar by providing the dismissable property, the sidebar still act as a modal, so the rest of the page is not interactive.

Sometimes can be helpful to have the sidebar open but still being able to access/use the test of the page.

Expected behavior

Being able to provide something like a asModal={false} property that do not render the p-component-overlay p-sidebar-mask overlay at all.

• PrimeReact version:
  2.0.0-rc-1