[GTK] Move implementation of isPlusMinusTreeClick() to Java layer

[ptziegler]

This exposes the following methods to Java land, allowing us to implement this method directly:

• gtk_tree_view_get_expander_column
• gtk_tree_view_get_cell_area
• gtk_tree_view_get_path_at_pos
• gtk_tree_path_free

This new implementation also avoids a potential resource leak because previously, the tree path was not freed properly.

See https://github.com/eclipse-windowbuilder/windowbuilder/issues/871