Let the Foot inset have a different Layout when inside a title

This allows to address two main issues
 * \thanks does only accept one paragraph, while \footnote allows several (ticket #2666)
 * footnotes in titling environments were not numbered on screen.

Moreover, the code reduces hardcoding of features, which is always a good thing.

There are several pieces in this commit:

 * new numbering type \fnsymbol for counters

 * the Foot inset changes its layoutName() to Foot:InTitle when inside a paragraph with InTitle property. This is set when running updateBuffer.

 * Foot:intitle uses the \thanks command, does not allow multiple paragraphs and marks its contents as moving argument.

 * The InsetLayouts for Foot now have properLaTeXName/Type, so that InsetFoot::latex can be removed; further code simplification is probably possible.

 * HTML preamble has been updated (thanks rgheck).

Fixes: #2666
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