Merge tooltip into master to replace Balloon with Tooltip

[RedFantom]

This branch contains changes that will improve the existing Balloon widgets and refactors it into the Tooltip widget. This branch is the tkinterpp-tooltip branch but rebased so that it only contains this widget and no other changes.

The parse_geometry_string overlaps functionality with a function introduced in PR #48 , so PR #48 should be merged before this one and then this one rebased on top of the new master so as to avoid having two functions which do the same thing. No tests are included for this function for this reason.

The changes in this PR are inspired by the functionality of @dogeek's tooltip widget. The new functionality includes:

• Additive binding to the master widget so as not to disrupt any existing bindings
• A new kwarg showheader to allow the user to hide the header of the tooltip
• A new kwarg static to allow static placement of the tooltip relative to the master widget rather than position relative to the mouse position within the widget
• A new kwarg offset to allow setting an offset from the position of the mouse or the master widget to show the tooltip

This is the PR for issue #39.

[sbordeyne]

Didn't have time to review until now, but I've reviewed it and I don't see anything wrong with this, so I approved for merging.

[RedFantom]

Thanks, @dogeek ! I will wait a few more days, as maybe @j4321 will also have time to review this. If not, then I'll merge it as is.