Allow user to control colors

[dennisrd]

Give the user control over the colors that are used in the minimap. There are multiple options, which can be combined:

• Fine-grained: the user determines for each value (in the table view) which color is used to represent that value.
• Color schemes: the user selects a "strategy" by which the colors are selected. For example, for the time stamps, it is helpful if small differences in time map to small differences in color, so that a smooth progress of time results in a smooth color gradient. For thread ids, it is more helpful if different numbers result in colors that are not similar, even if the ids are close together.

When colors are selected automatically, black should be excluded, because it may easily be confused with [no color].