An incorrect node is processed for some actions in the Tree Viewer

A context menu should contain actions for the node under the mouse cursor, not for the selected node.

Scenario to reproduce:

1. Open any tree.
2. Select any node.
3. Call a context menu on any other node.
   Expected state: if the "other" node is a root node, then "reroot" action is disabled.
4. Click any of the following actions: reroot tree, swap siblings, collapse.
   Expected state: the action is performed to the node that is under the mouse cursor.
   Current state: the action is performed to the selected node. If there is no selected nodes, then actions are disabled.