beautiful.tooltip_gaps and margins

[elig0n]

1. beautiful.tooltip_gaps description's usage of the word 'margins' in "The tooltip margins" is a bit confusing as it in-fact only refers to the distance between the tooltip and its parent widget while "true" inner-widget margins are defined via margins, margins_leftright and margins_topbottom.

2. Those margins properties could use beautiful variables since a theme designer would most likely want to have the same margins shared among most if not all of general tooltips instances.