Implement WebEngineError class for handling ErrorDomain

Change-Id: I6092483d46b8d0db6193b92f2744bd836beb4912
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Fix DownloadTargetHelper::determineDownloadTarget on Windows

filePathForCallback.AddExtension won't accept const char[]
with MSVC. It  needs to be converted with the appropriate function.

Change-Id: I95506de702eab4d7b61ddf051c7a122c793da35b
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Fix type conversion for Windows

MSVC uses WString instead of Utf16 so this needs different
conversion in type_conversion.h.

Change-Id: I987acb4ea5ad5a98c94fd992a9de05f602be825d
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>