Reuse existing FieldLineEdit widgets instead of re-creating them

A problem apparent with bibliographic entries with many co-authors is that
initializing the list of authors in the 'Edit Element' dialog takes long time.
One contributing factor is that a considerable number of QWidget classes get
created, initialized, and later destroyed, only to be created again under
certain conditions.

This commit tries to re-use existing widgets (FieldLineEdit) as far as possible,
by simply setting a new value to them. Superfluous widgets get destroyed and
new widgets get only created if necessary.

Loading entries with many authors is now faster, although not all performance
issues are resolved.

CCBUG: 379443