osgeo4w: add qt plugin directories to browser

Author: jef-n

ms-windows/osgeo4w/browser.bat.tmpl

@@ -3,4 +3,5 @@ call "%~dp0\o4w_env.bat"
 @echo off
 path %OSGEO4W_ROOT%\apps\@package@\bin;%PATH%
 set QGIS_PREFIX_PATH=%OSGEO4W_ROOT:\=/%/apps/@package@
+set QT_PLUGIN_PATH=%OSGEO4W_ROOT%\apps\@package@\qtplugins;%OSGEO4W_ROOT%\apps\qt4\plugins
 start "QGIS Browser" /B "%OSGEO4W_ROOT%"\bin\@package@-browser-bin.exe %*