ui-speedspacechart: speed limit tags enhancements

[Caracol3]

• adding logic to make the tooltip follow the cursor move horizontally and vertically
• sticking the tooltip to the chart when the cursor goes outside
• adding logic to prevent text going outside its tag Signed-off-by: Mathieu mathieu.coulibaly@sncf.fr

[Yohh]

there is a strange white border next to the EVO or MA100 tag :