When resetting FieldLineEdit, use preferred type flag

This commit addresses the issue reported in bug 417210,
comment 4, issue (d).

Among others, MenuLineEdit and, by extension, FieldLineEdit
got proper clear() functions and now a emit a modified()
signal which is different to the textChanged(QString) signal
as the modified() signal includes changes to the type as well.

This commit is a forward-port of commit 22ca454a from
branch 'kbibtex/0.9'.

CCBUG: 417210