Change wxDIALOG_NO_PARENT value to avoid clashing with wxCENTRE.

Change wxDIALOG_NO_PARENT value to avoid clashing with wxCENTRE.

Use a bit freed by removing wxTINY_CAPTION_HORIZ for wxDIALOG_NO_PARENT. This
allows to move it out from the lower byte of the style word to avoid conflicts
with the button selection flags which can be commonly combined with the dialog
styles.

More precisely, wxDIALOG_NO_PARENT used to clash with wxCENTRE, meaning that
wxSingleChoiceDialog, for example, was always created without parent because
its default style included wxCENTRE. This commit fixes this particular bug and
probably more similar ones.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64280 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775