BibEdit: refactoring and fixes

• Refactoring of functions related to editable area change

• Sets max characters on fields/subfields

• Avoids double bind of editable area. Was causing troubles.

• Fix bug in field tag modification undo

• Fix switch to write mode

• Fix merge with template. Record was not being ordered after merging with template, so operations on the recstruct where not being done correctly

• Change in css to always display scrollbar in webkit-based browsers.