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'
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