[FEATURE] Allow showing only one tip at the same time

This adds and option `discrete` which will, if set `TRUE`,
hide all other possibly visible Tips of the FloatingTips'
instance when showing a particular Tip.

As a consequence a member property `networkMembers` is
introduced which holds every current element the instance
is responsible for.

Fixes #19
Merge pull request #20 from wishbase/Issue19DiscreteTips

[FEATURE] Allow showing only one tip at the same time