Reverted DropDownList back to wxPopupWindow from wxPopupTransientWindow. The latter is not going to work, because it tries to be too smart. In particular, it thinks the mouse clicks outside the dialog when the scrollbar is clicked, and dismisses the popup. It also requires a mouse capture, which gives problems when submenus are used.

this design using wxPopupTransientWindow is probably not going to work, I should revert to the old version.

* fixed: popup didn't work when editor didn't have focus
* fixed: dimissal by losing focus didn't repaint dropdown arrows

use wxPopupTransientWindow::Dismiss instead of Show(false), otherwise handlers are no uninstalled and we get an assertion failure.

* cleaned up the interface of IconMenu a bit

* compatibility with wxWdigets 2.9+: Handle MOUSE_CAPTURE_LOST event
* use wxBORDER_THEME for custom controls

* overload wxEVT_ERASE_BACKGROUND, to reduce flicker
* use wxBORDER_THEME for custom controls
* tweaked style of GalleryList a tiny bit

* overload wxEVT_ERASE_BACKGROUND, to reduce flicker

* use wxBORDER_THEME for custom controls