Fix design of the playhead

[jbilcke-hf]

• 2px wide (fixed, it was due to a scaling bug)
• add a little playhead grabbing handle on top
• allow grabbing the playhead when hovering the line
• change the cursor on hover

usually editors just add a little triangle, I'd say it depends. I don't have strong opinion (imho it should be defined by the theme, that way if someone doesn't like it they can use a different theme)

can look like something a bit like this, with different width and heights: