add enter key event handler and focus style to open button in navigator

[kpge]

What it does

fix: #6140

1. add button:focus style when the button is focused (the same with button:hover)
2. hide more tool item on the toolbar in navigator when the workspace is empty.

How to test

1.open theia browser and close workspace
2.open Explorer sidebar, and press tab key to select open workspace button
3.press enter key to open open workspace dialog

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

[jbicker]

Works well! Thank you!

[akosyakov]

@jbicker don't forget to help with merging