Rework colormap dialog

[vallsv]

trafficstars

This PR improve some how the look end feel of the colormap.

This PR can be split if you prefer.

Gamma value

I would like to be able to play with the gamma correction value without thinking at the number it-self.

The gamma value is theoretically symmetric, from exp to log with a linear transfer function at 1.

The idea is to linearise the value between min/max range in order to localize the color of the lut which is at 128.

This value can be displayed in the histogram.

Refresh event

The colormap dialog and histogram is refreshed when the data change.

Unfortunately we don't have specific event, and most of the time the histogram don't have to be recomputed.

The PR tries to skip refresh when the colormapped data from the item is still the same (i.e. the same object)

This change have to be checked carefully, but i think it is fine while this array is created internally and should not be edited manually.

Changelog:

• Changed the colormap dialog layout
• Changed min/max autoscale check now integrated inside the edit text
• Added gamma value edition using a marker in the histogram
• Changed the refresh event to reduce computation
• Added example of image plot refresh using gevent
• Added local histogram when using setting fixed range