Add Tooltips to Sceen Tree

[Simon-Steinmann]

Especially for newcomers, many fields and settings in the scene tree can be foreign. Adding a tooltip area would make webots easier to get into.

Ideas on how to implement:

1. Show tooltip when hovering over an item in the webots gui and or scene tree
2. Add tooltips in the area below the scene tree, where fields can be changed (lower left area in screenshot)