Update refocus from menu actions, and adjust some options

[dpvc]

This PR is an update to update/explorer that fixes some problems with refocusing the selected item when the menu is use from within the explorer, particularly when the menu selection opens a dialog. It also moves the roleDescription option to the explorer's a11y options, so it can be configured even when the menu component is not loaded, and removes the internalSpeechTitles option, which is no longer supported since the speech is added to the DOM elements after the output is created.

The changes in explorer.ts make roleDescription an a11y option for the document.

In KeyExplorer.ts, we clear the nofocus setting after it is used, and change the button checking from button !== 0 to button === 2 when checking for right mouse buttons that will cause context-menu events.

In GeneratorPool.ts the setting of the options in the init() function is always done, so that if the menu changes enableSpeech or enableBraille and does a re-render of the output (which calls init() again), the options will be updated.

The changes to the SVG output jax are to remove the internalSpeechTitles option and the code that implements it.

The changes to the menu code is to better set the MathItem.outputData.nofocus and to set the speech explorer's refocus value to the node that was selected before the menu opened. Because there are focus changes that clear the selection before the contextmenu event is triggered, we use a mousedownevent to record the current selection before it is lost.

When a dialog is opened by a menu action, we need to keep the nofocus value, so introduce a new postInfo() method that records the nofocus value before posting an Info or SelectableInfo dialog box. This is used for all the .post() commands in the menu.

The roleDescription setting now uses a11yVar<string>() so we don't need to call setA11yVar() by hand in setRoleDescrition().

We add some checks to make sure options.displayOverflow and options.linebreak are available (since these are defined in output/common.ts, and may not be in all output jax).

We check for MathJax._.a11y.explorer rather than just MathJax._.a11y.speech or the other extensions, since we load explorer. This makes sure the explorer is available, not just the lesser extension.

We add code to tell the explorer where to refocus when the scale dialog box is open (it uses prompt() rather than an Info instance, so postInfo() isn't being called for this).

Finally, we remove the data-speech-node attributes that where introduced by the explorer update.

[zorkow]

lgtm.

With the exception mentioned in PR #1241, we solve those in the next PRs.