Allow expanding environment variables in XRC file paths

Add a new flag wxXRC_USE_ENVVARS for wxXmlResourceFlags that triggers a
call to wxExpandEnvVars() for bitmap, icon and animation paths.

This flag is not set by default to avoid silently changing the behaviour
of existing applications.

Closes https://github.com/wxWidgets/wxWidgets/pull/1445

interface/wx/xrc/xmlres.h

@@ -18,7 +18,14 @@ enum wxXmlResourceFlags
 
     /** Prevent the XRC files from being reloaded from disk in case they have been modified there
         since being last loaded (may slightly speed up loading them). */
-    wxXRC_NO_RELOADING   = 4
+    wxXRC_NO_RELOADING   = 4,
+
+    /**
+        Expand environment variables for paths in XRC (such as bitmaps or icons).
+
+        @since 3.1.3
+    */
+    wxXRC_USE_ENVVARS    = 8
 };
 
 
@@ -737,6 +744,16 @@ protected:
     */
     wxString GetText(const wxString& param, bool translate = true);
 
+    /**
+        Gets a file path from the given node.
+
+        This function expands environment variables in the path if
+        wxXRC_USE_ENVVARS is used.
+
+        @since 3.1.3
+    */
+    wxString GetFilePath(const wxXmlNode* node);
+
     /**
         Check to see if a parameter exists.
     */