Improve accessibility of modal dialogs (#1150)

[philippkraft]

Change aria-hidden attribute to aria-modal attribute to make modal dialogs accessible for screen readers

[philippkraft]

This pull requestis tested with JAWS on Windows. @neosonic2 will test this PR for two other major screen readers.

[neosonic2]

Testing this PR with other popular screen reading programs is taking slightly longer than anticipated as I am having problems with my local macOS development environment. I will comment again when I have that sorted out and have tested the PR with VoiceOver on macOS and NVDA on Windows, the other two most popular screen reading packages available besides JAWS.

[neosonic2]

I have finally been able to test out this PR using several other popular screen readers and below are my results. I tested this PR against the production version of Freescout used at my workplace, which is version 1.7.20. In both tests, I opened a conversation and clicked the Merge button in the More Actions menu to open that modal dialog.

When testing VoiceOver on macOS 12 Monterey against the production version of Freescout, it was able to recognize that the Merge Conversations dialog had opened, but the only thing it read out in the dialog was the Close button. No other elements were present.

However, this PR performed much better; VoiceOver was able to read everything in the Merge Conversations dialog without any issues.

NVDA 2021.3 running on Windows 11 produced similar results, except that with the production version of Freescout, NVDA couldn't see anything inside the dialog, not even the Close button. It detected that the Merge Conversations dialog had opened but reported that it contained no elements. Testing against this PR though yielded a dialog that was fully readable.

So I'd say this pull request accomplishes its stated goal of improving the accessibility of modal dialogs in Freescout across all major screen reading applications - JAWS for Windows, VoiceOver for macOS, and NVDA for Windows. I vote that it be merged into the upstream Freescout distribution.