Implement a UI for multiple phrase search

[timvandermeij]

PR #5579 implemented multiple phrase search, but as of now this is only accessible through the #search= URL parameter. It would be nice have a UI for this. We need to adapt our current find bar to add a checkbox for phrase search, but this also involves integration for the Firefox add-on.

[ribeaud]

See #7870 for corresponding pull request.

[Snuffleupagus]

Unless "phrase search" would also be generally implemented in the native Firefox browser findbar[1], this really cannot be implemented since it would result in inconsistent find UI behaviour.
Note in particular that for PDF files displayed in e.g. <iframe> tags the PDFFindBar implementation is still, by necessity, used even in Firefox.

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[1] The PDF Viewer in Firefox, which is built on the general PDF.js library, (normally) uses the browser findbar.

[Snuffleupagus]

Given that this, as outlined in #7368 (comment), is basically blocked on similar functionality existing for regular searching in Firefox itself, is it really meaningful to keep this issue open indefinitely?