Don't allow newline characters in document settings and preferences (#5840).

Don't allow newline characters in document settings and preferences (#5840).

Set a validator on QLineEdit widgets. The validator removes any
\n and \r characters, thus preventing users from copy-pasting
newline characters into these fields, and subsequently saving
them, inadvertantly, to their lyx file or preferences file.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/branches/BRANCH_2_0_X@39828 a592a061-630c-0410-9148-cb99ea01b6c8