fix: gui - highlight opcodes when selecting source

[kacperzuk]

What I did

Selecting any text just causes the selection to disappear, instead of highlighting relevant opcodes. This PR fixes this.

Related PR: #1512

How I did it

After releasing a button, SourceNoteBook._search() is triggered to find opcodes related to given source code. When searching for opcodes, it filters the pcMap with a term frame._label in v["path"]. v["path"] here is an integer index, while frame._label is a string path to the source file. As this compares apples-to-oranges, it always evaluates to false and triggers the code branch that clears selection.

In this PR I've changed the term to compare label to label.

How to verify it

I believe GUI has no tests, so you need to run it manually and select some source code.

Checklist

• [N] I have confirmed that my PR passes all linting checks - No, I'm getting mypy issues unrelated to my changes.
• [N] I have included test cases - No, this it not a new feature, it's a fix.
• [N] I have updated the documentation - ditto.
• [N] I have added an entry to the changelog. - No, what's the guideline here? It's not described in CONTRIBUTING.md