navigator: handle middle-click behavior

[vladarama]

What it does

Fixes: #12762
Fixes: #12788

This PR handles the middle-click button behavior in various tree-widgets, such as the explorer, search-in-workspace, references ... Like in VS Code, selecting a file using the middle-button will open that file as an open-editor instead of simply selecting it. This achieves the same functionality as double-clicking on a file, but it is faster.

Demo:

How to test

1. Start Theia
2. Open a Workspace / Folder
3. Go to the Explorer / Search-in-workspace / SCM /... views
4. Select a file by pressing the mouse's middle button (scroll wheel)
5. The file should be selected and added as an open-editor

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I noticed a bug when opening multiple editors using middle-click where we would incorrectly paste the clipboard into the editor (Linux):

1. start the application
2. confirm that content exists in the clipboard
3. open multiple editors from the explorer using middle-click
4. confirm the bug exists where clipboard content is being pasted into previous editors

[vince-fugnitto]

I confirmed that the changes work as expected 👍

• middle-click in the explorer, and other trees (ex: siw) works as expected
• middle-clicking collapsible nodes correctly expands/collapses nodes
• the auto-scroll behavior was fixed (confirmed on Firefox)

[msujew]

Works well for me as well 👍

[vince-fugnitto]

I'll merge, we can catch any potential regressions throughout the month if necessary.