MythUI: allow include files to be added to a window definition

This extends the current include file support to allow window definitions
to load additional widget definitions from an included file e.g.

<window name="playlistview" include="music-base.xml">
.
.
.
</window>

The main benefit over the current include support is the included file will only
be parsed when needed rather than every time an xml file is parsed.

mythtv/libs/libmythui/xmlparsebase.cpp

@@ -695,13 +695,17 @@ bool XMLParseBase::doLoad(const QString &windowname,
             if (onlywindows && e.tagName() == "window")
             {
                 QString name = e.attribute("name", "");
+                QString include = e.attribute("include", "");
                 if (name.isEmpty())
                 {
                     VERBOSE_XML(VB_GENERAL, LOG_ERR, filename, e,
                                 "Window needs a name");
                     return false;
                 }
 
+                if (!include.isEmpty())
+                    LoadBaseTheme(include);
+
                 if (name == windowname)
                 {
                     ParseChildren(filename, e, parent, showWarnings);