Generate default diagram name based on selected quantities (multi-line diagrams)

[ghorwin]

When creating a new diagram with multiple lines, generate the new diagram name using the quantity names of the selected variables, for example:

For single-file selections:

"T_1, RH_1 [$filename]"

For multi-file selections:

"T_1, RH_1"

[ghorwin]

Difficult to define default behavior - would need in-depth analysis, which is only available after extractors have been created. Defaulting to $filename for 2D and $project - quantity for 3D.