QWindowsXP/VistaStyle: Move the Vista treeview handling to QWindowsXPStylePrivate.

QWindowsXPStylePrivate maintains a list of theme handles for various
style items. The derived class QWindowsVistaStylePrivate had logic
to use a special helper window with the "explorer" window theme set to
obtain the correct treeview arrow branch indicator (arrow shape for Vista as
opposed to '+'/'-' on Windows XP) when creating the "TREEVIEW" theme.
This required calling the helper function
QWindowsVistaStylePrivate::initTreeViewTheming() before
QWindowsXPStylePrivate::createTheme(), which is prone to errors and
initialization order issues.

This could be solved by making QWindowsXPStylePrivate::createTheme()
virtual or similar, but since it the fate of QWindowsXPStylePrivate is most likely
to be merged into QWindowsVistaStylePrivate; it was decided to move
the entire special handling of the Vista treeviews into
QWindowsXPStylePrivate. The existing enumeration value
QWindowsXPStylePrivate::TreeViewTheme is renamed to
QWindowsXPStylePrivate::XpTreeViewTheme and a new value
QWindowsXPStylePrivate::VistaTreeViewTheme is added for which
QWindowsXPStylePrivate::createTheme() invokes the special handling.
This also removes the need to destroy the helper window in unpolish(),
which should save some initializations.

Task-number: QTBUG-52230
Change-Id: I0492ecf38fb3e5eabc4ecbdef70f0bf05e82e104
Reviewed-by: Adam Light <aclight@gmail.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>