Make the modal print preview window behave like a modal dialog.

Make the modal print preview window behave like a modal dialog.

It's unexpected for a modal dialog window to show up in the taskbar as
app's another window. It also shouldn't be possible to minimize a modal
window, because it's very confusing when a window is unresponsive
without a clear reason, because the modal child is hidden in the
taskbar.

Set wxFRAME_NO_TASKBAR and remove wxMINIMIZE_BOX to fix this. Do it only
for wxPreviewFrame_AppModal.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72301 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775