[trunk] Extensions to frame properties blg box (with patch)

Hi,

the idea was to extend the current dialogue box for frame properties to add the following:

1) frame height (for VBOX) or frame width (for HBOX) can be set numerically (for more precise results)
2) multiple selected frames changed with a single dlg box operation (useful for typographical consistency)
3) changes of frame properties added to undo stack

The attached patch does all the above (if multiple frames are selected, only those of the same type of the right-clicked frame are changed).

Local alpha tests were positive, more testing is welcome, of course!

Thanks,

M.