Truncate multiline by default in KiwiEntry

This is will accept only the first line when pasting a multi line string in
an entry.

It will be particularly usefull for users that copy and paste from a
spreadsheet, since a cell copied from there always has a new line and the
end.

Change-Id: I1acaa25dabf0b57773ccb05f01a2540c0fefb884