use of codicon icons where possible

[GitMensch]

This is "kind of" a draft. The main reason is that some symbols don't have an exact or "normal" matching icon, so I've looked up some possible alternatives to look at - and in the end the screenshots should be adjusted before a release can be done.

Reasons why those icons are preferable to the use of own svg files include:

• don't get broken when vscode does UI changes like it happened with #54
• do match icons used in other places of vscode - more streamlined and less confusing UI
• can be styled by theme authors and in general "match better" when using themes
• decrease of extension size

Note: I'm totally aware that when this extension started there was no codicon option to use. But nowadays those code icons are available in even old installations of vscode so I think it is better to use them.

[GitMensch]

kindly requesting review from @GorvGoyl

[GorvGoyl]

Thanks, @GitMensch I'm all in for built-in vscode icons. I hope all new icons look similar to the existing ones. Could you also mention the use of built-in icons in help.md (### Adding new buttons) file so that it's easier for new contributors. Once the PR is merged I'll create a new release.