XCB: Parentless modal dialogs may be hidden by parents

With Creator:

• Open git-based project, Choose extras/tools/git/reset or git show. Note the dialog is not transient (clicking on parent's title bar hides it below)

With the attached example:

• Click parentless dialog, note dialog is not transient

Analysis: XCB no longer creates a client_leader window which is set for transient windows that do not have a transient parent.