Open in Editor menu button is missing aria label

[timtmok]

System details:

Positron and OS details:

2025.01

Interpreter details:

Describe the issue:

The dropdown button is missing an aria label. A screen reader won't describe the element at all when it has focus.

Steps to reproduce the issue:

1. Activate a screen reader
2. Render a plot
3. Focus on the dropdown arrow on the Open in Editor button in the Plots view action bar

Expected or desired behavior:

A screen reader should describe that the element will open a menu to select where the plot will be opened.

Were there any error messages in the UI, Output panel, or Developer Tools console?

[timtmok]

Introduced from #5733