Improve wxSpinCtrl best size computation in wxGTK

Don't hardcode completely arbitrary width of 95px for the text part, but
compute it from the values this control is actually used for.

docs/changes.txt

@@ -76,6 +76,7 @@ wxGTK:
 
 - Implement setting link colours in wxHyperlinkCtrl for GTK+3 (Hanmac).
 - Support background colour in wxDataViewCtrl attributes.
+- Improve wxSpinCtrl best size calculation.
 
 wxMSW:
 

src/gtk/spinctrl.cpp

@@ -355,7 +355,16 @@ GdkWindow *wxSpinCtrlGTKBase::GTKGetWindow(wxArrayGdkWindows& windows) const
 
 wxSize wxSpinCtrlGTKBase::DoGetBestSize() const
 {
-    return DoGetSizeFromTextSize(95); // TODO: 95 is completely arbitrary
+    const int minVal = static_cast<int>(DoGetMin());
+    const int lenMin = wxString::Format("%d", minVal).length();
+
+    const int maxVal = static_cast<int>(DoGetMax());
+    const int lenMax = wxString::Format("%d", maxVal).length();
+
+    wxString longestText(wxMax(lenMin, lenMax), '9');
+    if ( minVal < 0 )
+        longestText.insert(0, "-");
+    return DoGetSizeFromTextSize(GetTextExtent(longestText).x, -1);
 }
 
 wxSize wxSpinCtrlGTKBase::DoGetSizeFromTextSize(int xlen, int ylen) const