Log Message:
tex-tcolorbox{,-doc}: update to 4.20

- bug fix: \multicolumn could not be used for first cell with option 'tabularx'

- internal colors names renamed and documented (Section 9.5 Color Names):
  'tcbcolframe', 'tcbcolback', 'tcbcolbacktitle', 'tcbcolbacklower',
  'tcbcolupper', 'tcbcollower', 'tcbcoltitle'

- listingsutf8 compatability better documented (#58)

- semantic changed for options: 'after upper*', 'after lower*'

- new options: 'before upper*', 'before lower*', 'titlebox', 'nirvana'

- library 'skins':
  * new options: 'beamer hidden', 'beamer alerted', 'hide', 'alert'
  * option 'only' moved from package to skins library

- new documentation section '13 Beamer Support'