Show full text in single line MSW wxTextCtrl if possible

A single line MSW wxTextCtrl created with a value too long to fit into
it continued showing the value only partially even if its size was
subsequently increased to allow the entire value to be shown.

This apparently happens because changing the native EDIT control size
doesn't affect its (horizontal) scroll offset. Moreover, there doesn't
seem to be any way to explicitly tell the control to update it neither,
except for changing its text.

So do change its text every time its width changes, as long as it is not
visible (because visible jumps in the visible text position could be an
even worse problem than the one we're trying to solve here). This fixes
the originally reported bug at the cost of a bunch of extra calls to
DoWriteText() which should hopefully be not too expensive for single
line controls that don't typically contain that much text.

Closes #18268.

include/wx/msw/textctrl.h

@@ -241,6 +241,8 @@ protected:
     virtual wxSize DoGetBestSize() const wxOVERRIDE;
     virtual wxSize DoGetSizeFromTextSize(int xlen, int ylen = -1) const wxOVERRIDE;
 
+    virtual void DoMoveWindow(int x, int y, int width, int height) wxOVERRIDE;
+
 #if wxUSE_RICHEDIT
     virtual void MSWUpdateFontOnDPIChange(const wxSize& newDPI) wxOVERRIDE;