keep parent canvas referenced from cloned instance

[Ant1r]

After creating a cloned instance, the reference to its parent canvas is currently cleared:

pure-data/src/g_clone.c

Line 213 in 8092921

retval->gl_owner = 0;

This PR proposes to keep this gl_owner reference intact, and use glist's field gl_isclone instead for discriminating clones when needed.

It allows a clone instance to behave just like any abstraction, and in particular it makes possible to:

• resolve [clone] misses [declare] -stdpath dependencies of main patch #929 and fix path problem using "clone" when editing the abstraction #410 (i.e clone cannot reliably use parents search paths)
• find the parent of the clone, through menu Window / Parent Window
• allow [pdcontrol] to retrieve parents information through [dir N( and [args N(

I basically replaced:
if (x->gl_owner)
with:
if (x->gl_owner && !x->gl_isclone)
where relevant.

(EDIT: I had previously forbidden [pdcontrol] to go back to clone's parent and retrieve its info, which wasn't necessary... I reverted that).