Fix #338 - add gt, gT support

[arussellk]

I did not write tests for this since the existing, closely-related actions MoveToRightPane and MoveToLeftPane did not have tests.

Let me know if anything looks wrong.

[rebornix]

I suppose this is about tabs, not editor groups, right? We are mapping editor groups to Vim's Window concept. For moving Tabs, you need to use other API, take a look at https://github.com/VSCodeVim/Vim/blob/master/src/cmd_line/commands/tab.ts . We've already had commands for them, and the logic is similar.

BTW, no worries about test cases of Tab, it's not testable with current test infra.

[arussellk]

@rebornix I rewrote my code to use the existing tab stuff. Check it out.

[johnfn]

LGTM! Thanks for the PR, @arussellk!