Button for showing PDF if compilation error

Button for showing PDF if compilation error

Building on cd8be655, we still allow viewing a produced PDF even if
there were compilation errors. However, now the user must click the
"Show Output Anyway" button in the LaTeX Errors dialog. The reason
is that before, there was a chance that the user would not realize
there was an error (because the PDF would be shown over the error
dialog). The approach in this commit makes it more clear that there
is an error.

A new LFUN is introduced, LFUN_BUFFER_VIEW_CACHE. It is useful not
just for the implementation of the "Show Output Anyway" button, but
also to show the last compiled version of a document, which can save
time if a document takes a long time to compile (e.g. heavy use of
knitr).