Resolves: fdo#79360 impress hangs on using sidebar new style

because those styles are "pseudo-styles" and a new one cannot be
added. The possibility is supposed to be disabled, and it is
disabled in the floating stylelist. The old code assumes there
can only be one of these stylelists and when a stylelist
queries if the "new" should be disabled the callback asks
the stylelist what family is selected, but only asks the floating one.

So, floating closed, sidebar open, the new is not disabled.
Implement the ancient TODO now that we have to. Instead of asking
the stylelist what family is selected, query the frame for what
is the current SID_STYLE_FAMILY as set by whatever is the active
stylelist.

What's disturbing is the SID_STYLE_FAMILY values are not SfxStyleFamily, but
indexes that have to be mapped to SfxStyleFamily. I bet there are a pile of
bugs around that, especially with little islands of different conversion
codesites

Change-Id: I85c8032d7c26ae6eea245685748f89b2a860e767