Colormap axis in InstrumentView mouse dragging improvements

[mantid-roman]

This issue was originally TRAC 3807

In log10 mode changing high values is too slow and low values is too fast.

[mantid-roman]

@NickDraper (2012-01-09T09:43:46):
Moved to iteration 33 at iteration 32 code freeze

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@NickDraper (2012-04-30T14:15:22):
Moved at end of release 2.1

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@NickDraper (2012-08-10T12:45:09):
http://www.mantidproject.org/Release_Notes_2_2_Full_List

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@NickDraper (2012-10-28T11:39:24):
Moved to milestone 2.4

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@NickDraper (2013-01-28T09:23:28):
Moved at the code freeze for release 2.4

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@NickDraper (2013-04-29T09:50:04):
Moved to r2.6 at the end of r2.5

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@NickDraper (2013-07-26T13:59:10):
Moved to the Backlog after the code freeze for R2.6

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@NickDraper (2014-02-14T11:07:56):
bulk move to assigned at the into of the triage step