feat: New method DownloadResponse::inline()

[iRedds]

Description
There is currently no way to change the Content-Disposition header when downloading a file, as this header will be overwritten when generating headers in the DownloadResponse class.

This PR prevents the Content-Disposition header from being changed when generating headers if that header has already been set.
It also adds a DownloadRespone::inline() helper method that sets the Content-Disposition: inline header.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[datamweb]

Nice feat!

[kenjis]

Thank you. This seems good. Please update the user guide a bit.

[kenjis]

LGTM!