PDFView: Don't let slider get too small
PDFView: Don't let slider get too small

Otherwise both TOC and  Content sliders get
too small, to the point is hard to spot and
to use.

https://phabricator.endlessm.com/T23215

endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib,endlessm/eos-knowledge-lib