Add feature $show_on_hover

[dov]

I suggest adding a new command to giv called $show_on_hover. It can be used as follows:

$show_on_hover 

Without any parameters, this means that the dataset will be shown whenever the mouse enters any painted pixel of the dataset.

$show_on_hover path=foo/bar/baz

With a path parameter, the dataset will be shown whenever the referenced path is hoverd over.

$show_on_hover 100 100

With an x/y coordinate but without a radius parameter this means that the dataset will be shown when the mouse appears in a default radius around the above point.

$show_on_hover 100 100 radius=10

This means that the dataset will be shown whenever the mouse is in a radius 10 around the datapoint.