For a program I'm looking to develop I need to be able to allow the user to add/remove/reorganize widgets.

I checked both fltk and qt and, while they do have drag and drop functionality available, that functionality seems to only be available to data contained in widgets and not the widgets themselves.

Now, I could just respond to the left mouse button down event and manually follow the mouse around with the widget, but would it be possible to move a widget from one container to another (no need to change from one window to another, just another box) or would mouse down automatically trigger the drag event on the data the widget holds?

I do like fltk's simplicity, but does that simplicity affect its usability?

About the only thing that comes to mind for draggable widgets is docking toolbars. Many toolkits provide some form of them.

As for toolkits themselves I'm currently enjoying gtkmm. fltk is very light but does leave a lot up to you that other toolkits don't. My current preference for gtkmm is that it allows me to use C++ constructs without qt's heavy use of macros. qt is certanly deep though.