Force WindowCloseButtonHint for toplevel dialogs

Prior to 6f6603eb, the close button had
been implicitly added by QtXCB. For example, if window flags contain
only Qt::Dialog, QtXCB will set MWM_FUNC_ALL motif hints. So QtQuick
dialogs used to get the close button even though there's no
Qt::WinodwCloseButtonHint flag. QtWidgets dialogs usually always pass
Qt::WindowCloseButtonHint flag.

After making kwin display minimize and maximize decoration buttons only
if corresponding Motif hints are set, KDE applications that use windows
with Dialog type lost their min/max buttons because QtWidgets code
doesn't set those flags.

6f6603eb tried to fix that by making the
platform integration theme force Qt::WindowMinimizeButtonHint and
Qt::WindowMaximizeButtonHint flags. But it accidentally broke QtQuick
dialogs since dialogs have additional flags besides Qt::Dialog now, so
QtXCB has stopped setting MWM_FUNC_ALL hints anymore.

In order to address that, this patch changes a few things:

- Make the platform theme be more careful about changing window flags.
  If user has explicitly changed window decoration flags, i.e.
  Qt::CustomizeWindowHint flag is set, don't touch decoration hints.
- Merge Qt::WindowMaximizeButtonHint with Qt::WindowMinimizeButtonHint.
  If the window cannot be maximized, the window manager will hide the
  maximize button, so we can pass Qt::WindowMinMaxButtonHint. In some
  way, it's a breaking change as child dialogs may not have maximize
  button, but it's a weird case and maybe user code should be in control
  of that?
- Forces Qt::WindowCloseButtonHint in addition to min/max button hints.

QtQuick child dialogs, i.e. transientParent() is not null, will get
default decoration buttons.

BUG: 464193
(cherry picked from commit 522824e4)