Add tooltips for warning lines

[TX256]

The new highlighting of warning lines is a great usability addition.

However, when one encounters a yellow line by scrolling, the only way to figure out what it actually means, is to open the Errors-panel or click the Warnings-icon and find out from there.

It would be nice to have a tooltip for lines that would display the actual warning.

[UnTabouret]

The same for errors would be pretty nice too

[Calinou]

Thanks for the proposal! Consolidating in #1005.