Slice Viewer: provide better interaction and expose position update event

[mdoucet]

This issue was originally TRAC 5317

The slice viewer should show a crosshair following the mouse position and provide event hooks to provide the current position to other widgets.

[mdoucet]

@mdoucet (2012-05-14T15:42:18):
Re http://trac.mantidproject.org/mantid/ticket/5317 Add crosshair
28c3444

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@NickDraper (2012-08-10T12:43:48):
Moved at the end of release 2.2

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@NickDraper (2012-10-28T11:38:58):
Moved to milestone 2.4

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@NickDraper (2013-01-28T09:23:08):
Moved at the code freeze for release 2.4

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@NickDraper (2013-04-29T09:49:43):
Moved to r2.6 at the end of r2.5

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@NickDraper (2013-07-26T13:54:59):
Moved to backlog at the code freeze for R2.6

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@NickDraper (2014-02-14T11:04:57):
Bulk move to assigned at the introduction of the triage step