Implement Nuke's `lib.read`

[davidlatwe]

Motivation

The Avalon data read-write methods implemented in Nuke was driven by prefixing knob name so to differ which knob is node's default and which is user-defined.

But the prefix may change, and the logic was not the same as other hosts, so if there's a way to differ user-defined and default knobs without prefixing knob name, I think we should opt that in.

Changes

• For unifying host implementation and pattern, adding lib.read to retrieve user-defined knobs from any given node, just like in other hosts. For layout type knob, like nuke.Tab_Knob, divider knob (nuke.Text_Knob with empty string value), and unnamed knob will be ignored.

• Backward compatible for knobs which prefixed with "avalon:" or "ak:". (Resolved this comment)

• Added flags optional arg on lib.Knobby for setting knob flag. Current read-only string data imprinting was implemented by using nuke.Text_Knob. But nuke.Text_Knob could be a layout divider if the initial value is an empty string, which could be confused with other read-only string data while reading. This change reduced the confusion by enabling knob flag setting while imprinting (e.g. nuke.String_Knob + nuke.READ_ONLY) .

[davidlatwe]

Just patched my motivation in above comment :point_up:

And, in the following GIF you may see the difference between divider and text knob, the string knob and read-only string knob.

Note that the string knob with nuke.READ_ONLY applied, user still able to select and copy string, but not with text knob.

[tokejepsen]

Looks good! Nice work @davidlatwe

[davidlatwe]

This will be merged tomorrow if no other objections :rocket: