NSIS: Fix multiple issues.

Restore the "!include qt-dll-manifest.nsh" clobbered by ga722f83 and
which resulted in missing DLL warnings (e.g. QT5PrintSupport.dll).

Fix overly aggressive search+replace in wireshark.nsi which broke the
WinPcap installation page and GTK+ selection. Try to finish the work on
promoting Qt started in ga722f83.

Bug: 10349
Change-Id: I7086be7c5272f4ae1b2cc26bbd5d5b725f013573
Reviewed-on: https://code.wireshark.org/review/3428
Reviewed-by: Gerald Combs <gerald@wireshark.org>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>