Ensure that we only increase the dialog width but never decrease it to
prevent the dialog from jumping annoyingly to different size with each
label change.
The previous attempt to do this in 4101849b4d (Grow
wxGenericProgressDialog automatically if needed, 2017-10-27) didn't work
because it seemingly didn't take into account the fact that wxStaticText
width changes when its label changes and so calling SetLabel() could
make it smaller, meaning that the next update would call Fit() again.
So fix this by preventing the control from changing its size by using
wxST_NO_AUTORESIZE and then only resizing it manually if it really needs
more space.
See #10624, #23727.
97546a6f0c