Advance tooltips: text selection

Be able to select the text of TXM tooltips

see: https://git.eclipse.org/c/ease/org.eclipse.ease.core.git/tree/plugins/org.eclipse.ease.ui/src/org/eclipse/ease/ui/help/hovers/internal/JavaDocLikeHover.java?id=812d16ce6155962034d6a95cdadcd5fa62d12dcc

(from redmine: issue id 2913, created on 2020/10/12 by Matthieu Decorde)