_tooltip.scss: replaced hard-coded 85% with `$tooltip-max-width`

tooltip show become too wide on large screens, since their `max-width:
85%` refers to the whole width of the screen. I have added a parameter
`$tooltip-max-width` and chose a default of `300px`.
Merge pull request #5190 from karland/patch-2

_tooltip.scss: replaced hard-coded 85% with $tooltip-max-width