Fix for #7468 - Cleared all highlights when find bar is closed

[OmarShehata]

This is a fix for this issue: #7468

In the PDFFindBar_close function, I added a loop to go through and clear all the highlights from all the pages. I wasn't sure whether to do it directly in that function, or create a wipeAll function in pdf_find_controller.js, so I just went with the most straightforward option. Let me know if this needs to be reworked in any way or have any feedback!

I also fixed a small grammatical typo in the comments. "Once the textLayer is built" instead of "Once the textLayer is build"

[Snuffleupagus]

Thanks for submitting the patch!
However, as is, this is introducing inconsistent behaviour between closing of the PDF.js findbar, respectively closing the Firefox findbar (note that we're using the browser findbar in the Firefox addon/built-in version of PDF.js).
I think that we need to decide if we actually want to fix #7468, and if so the behaviour really needs to be consistent between the two findbar implementations.

[OmarShehata]

Thanks for the clarification @Snuffleupagus !

I personally think it makes sense to clear the highlights on closing the findbar, but that's probably I got used to Adobe Reader's behavior doing that. I'll this as is, as a reference for anyone until a decision is made.

[qwer1304]

In latest pdf.js fd242ad this issue is back:
Search for anything
Close the search bar

Searched text is still highlighted

[Snuffleupagus]

This was superseded by PR #10100; @timvandermeij mind closing this one?