Fix field widths in wxStatusBar showing a size grip in wxGTK

Account for the size grip in DoUpdateFieldWidths(), otherwise the last
field overlapped it.

docs/changes.txt

@@ -134,6 +134,7 @@ wxGTK:
 - Implement wxDataViewColumn::UnsetAsSortKey().
 - Fix not showing wxInfoBar with GTK+ 3 < 3.22.29.
 - Fix the build with glib < 2.32 (e.g. CentOS 6).
+- Fix field widths in wxStatusBar showing a size grip.
 
 wxMSW:
 

src/generic/statusbr.cpp

@@ -184,8 +184,12 @@ void wxStatusBarGeneric::DoUpdateFieldWidths()
 {
     m_lastClientSize = GetClientSize();
 
+    int width = m_lastClientSize.x;
+    if ( ShowsSizeGrip() )
+        width -= GetSizeGripRect().width;
+
     // recompute the cache of the field widths if the status bar width has changed
-    m_widthsAbs = CalculateAbsWidths(m_lastClientSize.x);
+    m_widthsAbs = CalculateAbsWidths(width);
 }
 
 bool wxStatusBarGeneric::ShowsSizeGrip() const