Continue adding support for user created subgraphs

[adamgerhant]

Closes #2043

Allows removing imports/exports

TODO: Renaming and reordering imports/exports

[github-actions]

Found Clippy warnings

Clippy Warnings/Errors

warning: this expression creates a reference which is immediately dereferenced by the compiler
   --> editor/src/messages/portfolio/document/utility_types/network_interface.rs:846:15
    |
846 |                 get_ports(&modify_import_export_click_target)
    |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: change this to: `modify_import_export_click_target`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow
    = note: `-W clippy::needless-borrow` implied by `-W clippy::all`
    = help: to override `-W clippy::all` add `#[allow(clippy::needless_borrow)]`

warning: `graphite-editor` (lib) generated 1 warning (run `cargo clippy --fix --lib -p graphite-editor` to apply 1 suggestion)
warning: `graphite-editor` (lib) generated 1 warning (1 duplicate)
warning: `graphite-editor` (lib test) generated 1 warning (1 duplicate)
    Finished `dev` profile [optimized + debuginfo] target(s) in 4m 42s

    Finished `dev` profile [unoptimized + debuginfo] target(s) in 41.31s

[Keavon]

Deferred code review comments/requests from Hypercube in the precursor PR to this one:

• Add merging nodes into a subgraph with Ctrl+M and basic subgraph signature customization #2097 (comment)
• Add merging nodes into a subgraph with Ctrl+M and basic subgraph signature customization #2097 (comment)
• Add merging nodes into a subgraph with Ctrl+M and basic subgraph signature customization #2097 (comment)