Allow statement connections to connect around block stacks during drags

[rachel-fenichel]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

Resolves

Fixes #2131

Proposed Changes

Allow statement inputs to connect around block stacks during drags.
Remove distinction between first statement connection and other statement connections.

Reason for Changes

See #2131

Test Coverage

Tested with the example in #2131, and with an if-else block to test the case with more than one statement input.

Additional Information

This makes it possible to connect on a statement input, but it's still not a great experience--the insertion marker is a little too stubborn about sticking around, so you have to drag away from the target and then back to switch which of the statement inputs you're connecting to.

[rachel-fenichel]

@alschmiedt fyi

[rachel-fenichel]

Actually, the stickiness was because there was a check in InsertionMarkerManager.shouldUpdatePreviews_ that was checking if the proposed closest connection was the same as the old closest connection, but didn't check if the local connection had changed. Fixed in the third commit.