Add an action to open the file to the annotation context menu

[markiewb]

Add an action to open the file from the annotation context menu.

Use-Case: I tracked an issue down to its commit via the "blame"-sidebar ("Show annotations" in the context menu of the line gutter) and now I like to document this culprit line in a bug-tracker. Thus I like to copy the URL (or open the browser window and then copy the URL) to this file at the given commit.

Make sure that

• the commit/revision ID of the current hovered line in the annotation bar is used
• the line number of the current hovered line in the annotation bar is used (I am not sure, if this is possible )

[ben-gibson]

This has been added in release 2.3.0.