Implement Sticky Tooltips

[julienw]

See previous discussions: #1442 #1470 #1469

My proposal here is much less ambitious: being able to copy paste what's in a tooltip.

To do this, I propose the following MVP:

• only 1 sticky tooltip is possible. When another one is made sticky, the previous one disappears.
• it's triggered with a shortcut (eg: "p" for "pin") or clicking where possible (the marker chart comes to my mind).

This should be easier to implement than the other proposals and make it possible to get some feedback.

┆Issue is synchronized with this Jira Task

[julienw]

Current idea: show the tooltip for the currently selected marker.