[poly] Add possibility to add some vspace to frame-subtitles
maybe simply a length, that can be increased, since with some fonts the line height is weird (TeX Gyre Adventor, for example), so a \vspace{.25em} is needed in \framesubtitle
maybe simply a length, that can be increased, since with some fonts the line height is weird (TeX Gyre Adventor, for example), so a \vspace{.25em} is needed in \framesubtitle