[COMCTL32] Bugs when setting the title of property sheets or wizards

Using wine's comctl32 (this happens also with latest Wine 11), changing wizard (or property sheet) titles is buggy in the following cases:

• All the wizard pages don't have a title, and a title is initially set to the wizard dialog during initialization (via a `PSM_SETTITLE` message): the wizard should be displayed with the specified title (OK on Windows 2003, 7...), but is shown without any title (on ReactOS/Wine).

• The first wizard page (or a later one) has an explicit non-empty title, and the others have none: once switching to that page, this title should be set to the wizard dialog, and when switching to the other pages that have no title, the old (non-empty) title has to stay, until another page with a (possibly different) non-empty title is reached.
  This works on Windows 2003, 7... but not on ReactOS/Wine.

These problems are traced to the following places:

• In PROPSHEET_CollectSheetInfoA/W():
  Aero wizards do use PROPSHEETHEADER::pszCaption, so it should not forcefully be set to NULL when initializing a property sheet wizard.

• In PROPSHEET_ShowPage():
  The wizard dialog title should be changed on page change ONLY if pszText is NOT the empty string.

• In PROPSHEET_DialogProc(), WM_INITDIALOG handler:
  The IS_INTRESOURCE(pszCaption) special case is useless, since PROPSHEET_SetTitleW() can deal with strings passed via resource ID.
  In addition, PROPSHEET_SetTitleW() should be invoked only for property sheets, not wizards (except for Aero wizards, which is a TODO).