Linguist: Fix MSVC-warnings about double truncation.

messageeditor.cpp:941: warning: C4305: '*=' : truncation from 'double' to 'float'

Change-Id: I1ddf5a9cc6aefaab1d62c33cc95955d981ea5fb9
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>