It wouldn't be too difficult to build on Glade or QtDesigner and better integrate it with code generation. Both Glade and QtDesigner output language-neutral UI descriptions that could be used to build any sort of app.
Lazarus already saves the forms into text-based files that you can read, however this is really missing the point and reducing an advanced RAD tool to nothing more than 80s-circa resource editor for Windows. There are tons of tools like that.
The big feature of Lazarus (and Delphi and C++ Builder, etc) is how integrated the entire thing is - not just the form designer but also the framework being itself designed around its use inside Lazarus' form designer, object inspector, etc and the language having features (e.g. published properties) explicitly for such use and the IDE knows about the framework so it can automatically plug things, generate code (not in the Glade style of unidirectional generation but in that the IDE understands the code and can modify it). Hell, even C++ Builder actually feels a bit "alien" to VCL (which was originally made for Delphi and Borland had to add several extensions to C++ for it to work, which only really worked because they had their own compiler, linker, etc to work with).
The closest you could get to that elsewhere is QtCreator but you quickly realize (assuming you were used to Lazarus-like tools, most developers actually do not realize that) you are working with something that was meant to be coded against instead of used with visual designers (which is kinda backwards IMO considering the "G" in GUIs and kinda remind me a quip about Motif back in the day about how its documentation was a bunch of volumes almost without any screenshot :-P).
But if you want to do it right, you can't really work around the need for all these to be integrated with a unifying vision. Even Lazarus and Free Pascal, which are technically different projects, largely managed it because they basically did what Delphi already had done - and most of the warts when using Lazarus come from not being a truly integrated project like Delphi was, like issues with different FPC versions or relying on GDB as a debugger (which is the #1 source of debugging issues, fortunately there is a new debugger in works which can work as part of the IDE itself and personally find it to work fine, though it isn't enabled by default).
