Add more suitable popup anchors fordeafult styles

Added a suitable anchor image for each default theme and created a
theme-specific popup file which can be included into the GeoExt-app.
set the classic theme anchor as default anchor
Popup-example loads the correct popup-CSS-file

The popup-example now loads the popup-CSS-file depending on the chosen
theme by the theme-switcher-combo. So the correct anchor is shown when
opening the popup in the example-app.
Merge pull request #257 from chrismayer/more-popup-anchors

Merge More theme-specific popup anchors (r @bartvde & @marcjansen)