Support display and connection of attributes that are part of GroupAttributes

[cbentejac]

Description

This PR is based on #2538 and must be merged once #2538 itself has been integrated into develop.
It also relies on some UI fixes introduced in #2563.

The main feature of this pull request is that it adds the support of connections between attributes that are part of a GroupAttribute. Prior to this PR, these attributes could neither be connected individually, nor be displayed in the Graph Editor; they were however visible and editable in the Node Editor.

GroupAttributes are now displayed in the Graph Editor with right chevron, which means they can be expanded when clicked upon. When this happens, all the children attributes of the clicked GroupAttribute are displayed below it, displayed in a tree view with the indentation reflecting their depth level. Children attributes are displayed recursively, and a GroupAttribute that contains a child GroupAttribute will have it displayed as well (but collapsed by default).

Attributes that are connected will always be displayed, even if they belong to GroupAttributes that are currently collapsed. If an edge connecting such an attribute is removed, then this attribute will be updated to reflect its parent's visibility status.

Additionally, some unit tests are added.

Features list

• Add the notion of depth for attributes that are children of a GroupAttribute. An attribute that has no parent will have a depth of 0, and it will increase of 1 for every parent;
• Add a flatStaticChildren property for every attribute that may have children that contains the flattened list of said children;
• Correctly evaluate links when they reference children attributes, hence adding the support of the connection of individual attributes within groups;
• For children attributes, have their parent's exposed property take precedence over their own. This for example prevents having attributes within groups that are exposed while the group itself is not, which would break the hierarchy of the display for these GroupAttributes;
• Add an icon to expand or collapse GroupAttributes and display their children attributes in their node in the Graph Editor with a tree view. These children attributes are graphically connectable;
• Display the full name of the attribute in its tooltip in the Graph Editor. If an attribute childAttribute is a child of a group groupAttribute, its name will be displayed as groupAttribute.childAttribute, making the relationship between the two obvious.

[cbentejac]

Need to check if the generation of the attributes within a group in the node cannot be performed in the same way as in the node editor.

[codecov]

Codecov Report

Attention: Patch coverage is 98.83721% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.38%. Comparing base (faff99f) to head (6aae86c).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
meshroom/core/node.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2544      +/-   ##
===========================================
+ Coverage    69.90%   70.38%   +0.48%     
===========================================
  Files          121      123       +2     
  Lines         7078     7173      +95     
===========================================
+ Hits          4948     5049     +101     
+ Misses        2130     2124       -6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.