Fix/explorer rules

[zorkow]

Reenables the change of rule sets and preferences for speech generation.

• Uses a standard variable definition forspeechRules.
• Switches to clearspeak as default, thereby also fixing a type mathspeek that lead to incorrect speech computation.
• Fixes an error when the selection box is closed, as it is not attached to a mathitem.

Still two issues:

• I still need to make an SRE release.
• Currently one can select rule sets that do not always exist for every locale. There are two solutions for this:

1. disactivating the menu items that should not be chosen, which would require quite a bit of refactoring of the current speech menu code,
2. on the other hand I always wanted to ensure that SRE produces reasonable output even with a non-existing rule set is chosen. I go with this solution for now.

[dpvc]

One change suggestion to simplify the speechRules definition, but other than that looks good.

[dpvc]

You should be able to use this.variable() rather than this.a11yvar() to define this, so you don't have to do the getter and setter by hand (and won't need to include lines 517 and 521, which are part of what this.variable() does. So

          this.variable<string>('speechRules', value => {
            const [domain, style] = value.split('-');
            this.document.options.sre.domain = domain;
            this.document.options.sre.style = style;
            this.rerender(STATE.COMPILED);
          }),

should do it (that is, changing a11yvar to variable and adding the rerender() call to the original lines 510 to 514 should be enough).

[zorkow]

I tried that and it not work. And it drove me crazy, so I decided to stick with the pedestrian version.
But I will try again. Maybe I was just too thick the other day.

[dpvc]

I just tried this out, and it seemed to work fine for me. Give it another try, and see if you have any problems.