Use more reasonable length for wxSlider in wxGTK by default

Use the same 100 DIPs as in wxMSW, to make the default behaviour more
useful.

Also update documentation to explain which size component should, and
should not, be specified when creating the slider.

Closes https://github.com/wxWidgets/wxWidgets/pull/2012

src/gtk/slider.cpp

@@ -571,6 +571,16 @@ int wxSlider::GetTickFreq() const
 #endif
 }
 
+wxSize wxSlider::DoGetBestSize() const
+{
+    // We need to get the size in the transverse direction from GTK, but we use
+    // hard-coded default in the other direction, as otherwise the slider would
+    // have the smallest possible size and not have any extent at all.
+    wxSize size = GTKGetPreferredSize(m_widget);
+    (HasFlag(wxSL_VERTICAL) ? size.y : size.x) = 100;
+    return size;
+}
+
 GdkWindow *wxSlider::GTKGetWindow(wxArrayGdkWindows& WXUNUSED(windows)) const
 {
 #ifdef __WXGTK3__