Further improve best size of wxDatePickerCtrl with wxDP_ALLOWNONE

Compute the best size more precisely to be exactly compatible with the
control appearance when not using wxDP_ALLOWNONE. This unfortunately
requires hardcoding an arbitrary constant in DoGetBestSize().

Patch used for testing this code for future reference:

diff --git a/samples/minimal/minimal.cpp b/samples/minimal/minimal.cpp
index 470e765423..a1d5fb9938 100644
--- a/samples/minimal/minimal.cpp
+++ b/samples/minimal/minimal.cpp
@@ -26,6 +26,8 @@
     #include "wx/wx.h"
 #endif

+#include "wx/datectrl.h"
+
 // ----------------------------------------------------------------------------
 // resources
 // ----------------------------------------------------------------------------
@@ -175,6 +177,28 @@ bool MyApp::OnInit()
     CreateStatusBar(2);
     SetStatusText("Welcome to wxWidgets!");
 #endif // wxUSE_STATUSBAR
+
+    auto sz = new wxBoxSizer(wxVERTICAL);
+    auto addDPC = [this, sz](int style)
+    {
+        sz->Add(new wxDatePickerCtrl(this, wxID_ANY, wxDefaultDateTime,
+                                     wxDefaultPosition, wxDefaultSize, style),
+                wxSizerFlags().Right());
+    };
+
+    addDPC(wxDP_DROPDOWN | wxDP_ALLOWNONE | wxDP_SHOWCENTURY);
+    addDPC(wxDP_DROPDOWN | wxDP_SHOWCENTURY);
+    addDPC(wxDP_DROPDOWN | wxDP_ALLOWNONE);
+    addDPC(wxDP_DROPDOWN);
+
+    sz->AddSpacer(10);
+
+    addDPC(wxDP_SPIN | wxDP_ALLOWNONE | wxDP_SHOWCENTURY);
+    addDPC(wxDP_SPIN | wxDP_SHOWCENTURY);
+    addDPC(wxDP_SPIN | wxDP_ALLOWNONE);
+    addDPC(wxDP_SPIN);
+
+    SetSizerAndFit(sz);
 }

src/msw/datetimectrl.cpp

@@ -207,7 +207,12 @@ wxSize wxDateTimePickerCtrl::DoGetBestSize() const
     // size when using DTS_SHOWNONE than when not using it, which doesn't make
     // any sense at all).
     if ( MSWAllowsNone() )
-        size.x += 3 * GetCharWidth();
+    {
+        // The extra 10px here was determined heuristically as the value which
+        // results in the same layout with and without DTS_SHOWNONE under
+        // Windows 7 and Windows 10 with 100%, 150% and 200% scaling.
+        size.x += wxGetSystemMetrics(SM_CXMENUCHECK, m_parent) + 10;
+    }
 
     int scrollY = wxSystemSettings::GetMetric(wxSYS_HSCROLL_ARROW_Y, m_parent);
     size.y = wxMax(size.y, scrollY);