Set initial window size as its minimal size.

Set initial window size as its minimal size.

The change of r62306 broke the code which relied on the window created with a
given initial size to never become smaller than this size. This was to be
expected because the best size of such window is now (1, 1) and not its
initial size but we still need to preserve the old behaviour for compatibility
and also because it usually does make sense to avoid shrinking the window with
an explicitly specified initial size beneath this value.

So call SetMinSize() with the initial size explicitly now in wxWindowBase
ctor.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62814 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775