Properly hide the suggestions list in some cases.
Revert "Use the same window flags for the scan window, on Win and Linux."

Unfortunately, the change didn't work properly on KDE systems.

This reverts commit 8a8c7cc7d7356495bf3ae53c0f01a266143e49ee.
Make the suggestion list to be internal instead of top-level window, on *nix.

This should solve the conflict between the Qt::Popup-styled Scan window and
the suggestion list.

Tested on Windows 7, Lubuntu (XFCE) and Mint (KDE).
Fix for the suggestion list on Linux, classic UI.