Clean up the .merlin

I added a .merlin in ide/ who inherits everything from the root .merlin and also
adds the dependency to lablgtk, which I removed from the root file. These way
people not working on that part of the code won't get bothered if they don't
have that package.

I removed the S/B entry for plugins which was useless, indeed there is no ML
file in that directory and merlin doesn't scan the subdirectories recursively
(as you know).

I also removed the S/B entry for checker since most of the files of this
directory are also present in kernel and that was the cause of a lot errors on
merlin's side (think "inconsistent assumptions").
On top of that, no part of the tree depends on checker (I back that assertion by
a grep of the *.d files of the tree) so these lines in the .merlin were actually
useless. The only part of the tree where you need to know what's in checker/ is
when you are working in checker/ itself, but since merlin automatically adds the
directory of the file under edition in its source and load paths nothing else is
needed.

There might still be problems after this patch, but they should be less of them.
Considering my poor knowledge of the codebase there might be other conflicts I
have missed.