ModEdit: fix some issues due to the fact legacy footprint libraries cannot handle 32 copper layers, and are therefore no more editable:
* in file/library save as..., remove option to save a lib under the legacy format (which is not possible and creates an error message)
* if the current loaded lib is a legacy lib, when the user try to save or delete a footprint, displays a message which explains what the user should do.
* add a suitable dialog to select/create a .pretty folder library (a file dialog or a dir dialog coming from wxWidgets is not friendly usable.)