Separate virtual buffer pass through mode for each buffer

[jteh]

Currently, there is only one global virtual buffer pass through mode. This is fine if one is working with only one document. However, when working with multiple documents, it is often desirable to have different active modes for each document. For example, one might wish to fill in a form in one document while simply reading another, with the appropriate mode preserved for each document. Currently, one must constantly manually enable and disable pass through mode when switching documents.

Speaking the current mode whenever one switches to a document is undesirable. Ideally, the mode should only be spoken when switching documents causes it to change. Thus, a cache of the last mode needs to be kept and the mode will only be spoken if the cached value is different to the mode for the current buffer. In terms of implementation, there will probably be a function such as virtualBufferHandler.reportVirtualBufferPassThroughMode which is called on each gainFocus event. This will be responsible for maintaining and checking the cached mode and speaking it as appropriate.

[jteh]

Implemented in r1861.