Review and tag qtbase/src/gui/kernel/*

The task is to review all code mentioned in the task title for the presence of code that is considered security-critical, according to https://contribute.qt-project.org/quips/23. In particular, we are looking for parsers that touch untrusted data directly. Such code should have the security header

// Qt-Security score:critical reason:data-parser

Definition of done:
1. All code mentioned in the task header is reviewed
2. If security critical code is found, the header is added.

Note: This is a bit fuzzy, but I think it mostly concerns about processing of untrusted data from DnD and clipboard.