modernize labels

Labels in ParaView are cumbersome to use, inexact, and slow. End users enable labels on the data within selections and further subsampling occurs behind the scenes to limit the number of labels. Colormaps by contrast are a simple pull down in the active variables toolbar.

We should add a new label interface that starts as a simple "Draw Labels" checkbox in the display tab to make it really easy for the user to add labels to any object in the pipeline. The user should be able to pick from any of the cell or point arrays as they can for colormapping. The use should have some control over the subsampling algorithm that goes on to reduce clutter and overlap. The user should be able to control the font characteristics as well.

Existing capabilities to label selections should remain with an eye toward unifying the UI between old and new style labels.