Bug #4730 / InvalidDate-00-00-2000 FE.type=date - detection of empty date was broken for '00.00.0000'.

AbstractBuildForm.php: new function subrecordHead(). Replaced several hard coded subrecord column names against constants.

# Conflicts:
#	javascript/src/QfqForm.js

Feature: Upload Button - wrapped with Bootstrap Button - reverted to old style, delete/refresh behaviour broken with bootstrap style

#4606 / _link: qualifier to render bootstrap button - add tooltip to button/text, even if there is no link

#4606 / _link: qualifier to render bootstrap button - implement token  'b:...' for link class. Manual is updated. Open: `pageX` should be recoded to use the new 'b:' instead of hardcoed behaviour to render a button.

Feature: Upload Button - wrapped with Bootstrap Button. New option 'fileButtonText' to specify a button text.

Feature #3752 / Pills auf mode|modeSql=hidden|readonly setzen - implemented during 'form load' (not dynamic update)