Visualiser mode plug

[ericmehl]

This adds a new plug to the VisualiserTool for the mode of visualisation. Right now we're still only displaying color visualisation, but other types will be coming shortly. The current modes are :

• Auto : For now the same as Color (Type Range). In the future this will change the way various data such as vectors and integers are displayed. I'm including it now as I expect it to be the default in the future and including it now prevents a change in default behavior when Auto becomes more meaningful.
• Color (Type Range) : Displays like the existing color visualisation, but the range changes based on the type of data. Data is unmodified except vectors, which are remapped from [-1, 1] to [0, 1].
• Color (Manual Range) : This is the existing mode where you get valueMin and valueMax plugs and values are remapped from [valueMin, valueMax] to [0, 1].

I considered naming "Color (Type Range)" as "Color (Auto Range)", but went with "Type" because we briefly talked about a mode where we look at the min and max value of the data and use that for the range. That seems more "Auto"-like to me so I figured I'd try not to preclude that future use.

Depends on #6181.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[ericmehl]

I pushed a rebase onto 1.5 to pick up the changes from #6181 that this depends on. I think the commit on this PR might be included in an upcoming PR introducing per-vertex label visualisation (which depends on this commit), but for now I'm just keeping this up to date.

[ericmehl]

Closing as this is now included in #6212.