* configure.ac.in: bail out with an error message if the x11 skins are enabled and we couldn't find imlib2.
* modules/gui/wxwindows/*: fixed bug in sout dialog box.
* modules/gui/skins/*: factorised the wxwindows dialogs code in dialogs.cpp. wxWindows is also initialised earlier on to allow opening a skins file with a wxWin dialog box. (Note: I badly broke the GTK2 skins).