Support codicon icon references for plugin view containers

[colin-grant-work]

What it does

Fixes #10490 by implementing handling of $(icon-name) strings for plugin view containers.

How to test

1. Change the version of vscode-references-view from 47 to 81 in the builtin plugin manifest in package.json
2. Delete any existing versions of vscode-references-view from your plugins folder and run yarn build
3. Start the application with the Theia repo as your workspace.
4. Find an interesting variable and run Find All References or find an interesting function and run the command palette command Calls: Show Call Hierarchy
5. Observe that the vscode-reference-view plugin view appears, and the plugin's view container has an icon and that that icon is styled like the rest of our Codicon view container icons.

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

Signed-off-by: Colin Grant colin.grant@ericsson.com

[colin-grant-work]

@msujew, since you've had a good look at various bits of Codicon related machinery recently, would you mind checking this out?

[msujew]

Looks good to me 👍

• The codicon is correctly inserted into the class list for the tab icon
• The theme icon is visible in both sidebar and tab-item mode
• The svg-mask is not used when a themeIcon is present
• Using masks still works correctly as expected (I used the Docker extension to test this)

Code looks reasonable, no complaints there.