Timehistogram labeling

[rsimon]

This can wait for later, but: the labeling of the time histogram isn't ideal. Probably needs a more fundamental redesign.

• z-index order of the handle labels vs. the magnifying glass icon is broken
• handle labels have a fixed size, label occasionally exceed it
• when the select interval is narrow, handle labels overlap each other
• 1 AD label can overlap the min/max year labels
• general color could be improved