Load footmisc.sty instead of using outdated code

This is a patch I originally sent to lyx-devel in 2012 with subject
'Load footmisc.sty instead of using copied code from obsolete stblftnt.sty'.
It now takes all comments into account: It works also if the user loads the
package herself, it can be disabled by providing the footmisc feature in a
layout, and it does not use the ugly \AtBeginDocument{}.
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.11.14-20.00