[GTK] Optimize TreeItem constructor by removing ID system eclipse-pla…

…tform#882

The ID system tracking association of every TreeItem with GTK model
entry had been using GTK.gtk_tree_store_set() to persist a key in GTK
model. This operation has linear execution  time over model size.

IDs were replaced with tree paths and strong references, which, if
implemented right, have logarithmic execution time on balanced trees and
constant execution time on wide trees.

Performance improvement is proven by running
org.eclipse.swt.tests.junit.performance.Test_org_eclipse_swt_widgets_Tree.jfaceReveal()

In test jfaceReveal[Shape: STAR, virtual: true]:
10_000 elements:  141_061_117ns -> 4_369_889ns
100_000 elements: 10_761_449_641ns -> 181_898_611ns (-98%)