Use a QLabel for scrollbar linenumbers tooltip

QTooltip sometimes hangs off the corner of window and flickers which is very annoying. We can use a QLabel here instead, which means a much faster and more responsive tooltip that shows up when we want and hides when we tell it to go away.

The behaviour is such that:

• show tooltip when mouse is pressed and moving on scrollbar
• hide immediately and always when mouse is released. This results in slightly changed logic (the if condition is gone) because it wasn't getting hit and thus the tip would stay flickering