Add support for using arbitrary windows as wxStaticBox labels

This commit implements the new feature in wxGTK and updates the sample
and the documentation.

docs/changes.txt

@@ -113,6 +113,7 @@ All (GUI):
 - Allow wxWebView::RunScript() return values (Jose Lorenzo, GSoC 2017).
 - Allow using fractional pen widths with wxGraphicsContext (Adrien Tétar).
 - Add support for loading fonts from external files (Arthur Norman).
+- Add support for using arbitrary windows as wxStaticBox labels.
 - Improve wxSVGFileDC to support more of wxDC API (Maarten Bent).
 - Add support for wxAuiManager and wxAuiPaneInfo to XRC (Andrea Zanellato).
 - Add support for wxSL_MIN_MAX_LABELS and wxSL_VALUE_LABEL to XRC (ousnius).

docs/doxygen/mainpages/const_cpp.h

@@ -194,6 +194,8 @@ Currently the following symbols exist:
 @itemdef{wxHAS_RAW_KEY_CODES, Defined if raw key codes (see wxKeyEvent::GetRawKeyCode are supported.}
 @itemdef{wxHAS_REGEX_ADVANCED, Defined if advanced syntax is available in wxRegEx.}
 @itemdef{wxHAS_TASK_BAR_ICON, Defined if wxTaskBarIcon is available on the current platform.}
+@itemdef{wxHAS_WINDOW_LABEL_IN_STATIC_BOX, Defined if wxStaticBox::Create()
+    overload taking @c wxWindow* instead of the text label is available on the current platform.}
 @itemdef{wxHAS_MODE_T, Defined when wxWidgets defines @c mode_t typedef for the
     compilers not providing it. If another library used in a wxWidgets
     application, such as ACE (http://www.cs.wustl.edu/~schmidt/ACE.html), also