qt: Fix crash when moving a Component

[aleixpol]

We need to invalidate the one we are moving, otherwise we get a 2x
unref.

[ximion]

CI failures are unrelated. Thank you for the patch!

[ximion]

I may have merged this too quickly, as it looks like if you don't create a new component for the moved component (other.m_cpt = as_component_new () as well), then the other component will try to unref a nullpointer, which will either be a hard error or at least give a nasty warning (or am I missing something here?).

[aleixpol]

I checked and it does warn when it finds a null object to unref.

#379

[ximion]

Jup, sorry - should have actually reviewed this more carefully!