Feature request: Add indexing support

[mikolysz]

Some speech backends can indicate to the caller when a particular point in the requested utterance has been reached (or where that point exists in the buffer, when speaking to memory). This is particularly useful when reading larger text documents and when cursor tracking is desired.

This is a capability that Prism should expose to callers when it is supported by the backend.

The input side of this is basically implemented already (with SSML's mark element). All we now need is a way for backends to signal Prism that a particular SSML mark has been reached, and for that signal to be passed back to calling applications.

[ethindp]

@mikolysz Hi there,

Thanks for the request. I have been considering adding SSML support in general to Prism, although I have been weighed down by uncertainties about exactly how to go about it. Various backends support various SSML versions and grammars; for example, SAPI obeys SSML 1.0 but with Microsoft-specific extensions and regressions, MacOS supports a version of it that I'm not entirely sure about, and I have no idea what version Speech Dispatcher and Spiel supports (assuming either of them handle SSML at all and it isn't "let's just defer it to the underlying engine" which would complicate things even further). And in some cases, the SSML speech path and the ordinary one are identical.

I would like however to get to this at some point, as well as add a few other niceties as well; I admittedly have been distracted with other projects.