NIFI-7650: Add basic focus management to UI

[sardell]

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Description of PR

This PR adds some basic focus management to the NiFi UI. This helps those using assistive technologies or keyboards navigate interactive elements on a page. When testing, you’ll notice the following changes:


• A user is now able to tab through all of the interactive controls on the canvas page. This includes the components toolbar, global menu, navigate palette and operate palette.
• The global menu is now navigable via the up and down arrow keys.
• Focus is now trapped inside of modals as described in the wai-aria modal best practices document, as well as some shell dialogs (for example, Bulletin Board and Templates). Unfortunately, SlickGrid applies tab index to hidden elements it generates, which in turn breaks the code that traps focus inside of shell dialogs. This can be resolved by contributing to SlickGrid or switching to another package in the future. I created a Jira ticket to track the issue.
• I applied the same :hover styling to :focus where possible, but some elements (such as the breadcrumb and bulletin button) do not have :hover styling.

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[sardell]

Closing this PR in favor of smaller PRs that are easier to test.