Viewer slider

[paskino]

Adding a slider widget to control the slice that is being displayed. Both the mouse wheel and this slider can be used to change the slice.

A SliceSliderRepresentation with default colors (CIL pinks), orientation (horizontal) and position is introduced.

Closes #358

The orientation and position of the slider can be changed.

I tested on Windows with the web app and it doesn't seem to be able to grab the slider.

[paskino]

TODO:

• [x] Slider resets when changing orientation
• [x] Slider is reset when loading a new image
• [ ] Slider can be hidden

[DanicaSTFC]

I tested on Windows and magma data .npy and it seemed to work correctly.

[DanicaSTFC]

I changed the bar colour back to gray. The previous magenta was not giving enough contrast with the viewer blue background.

[DanicaSTFC]

I opened two issues to add features to this slider so we can close this PR and continue the enhancements at a later stage

381 #380