Minimize encoding conversions when generating unique file name

Minimize encoding conversions when generating unique file name

With minor adjustments, createFileFromTemplate is made to work directly
on (UTF-16) QString data, which is already in the native encoding for
Windows and Symbian. This is possible because the function only fills
out the placeholder sub-string, without touching adjacent characters.

This eliminates unnecessary conversions on those platforms.

Reviewed-by: Gareth Stockwell
Reviewed-by: Shane Kearns
(cherry picked from commit 9a76587363a2f37312326286e08cce502f7fe27e)

Change-Id: I8732b88ece5e2befb2da2e717758954c9aa7e5b0
Reviewed-by: João Abecasis <joao.abecasis@nokia.com>