Add commands to move to next/prev tab bar in shell

[qntnrbns]

References

Fixing #7434, so that users can toggle between Tab Bars

Code changes

added activateNextTabBar and activatePreviousTabBar to LabShell

User-facing changes

Users can now move though different panels using Ctrl Shift . to go the the next tab bar and Ctrl Shift , to go to the previous. There are also commands in the menu and command palette.

Backwards-incompatible changes

None

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[afshin]

I am 👍 on the implementation. I just want to tag @tgeorgeux to ask if we have any heuristic or convention about which keyboard shortcut to use for a command. Is this good to go on that front, too?

[jasongrout]

Looks like vs code has a command (workbench.action.navigateEditorGroups) to cycle between tab bars, but does not assign it a default keyboard shortcut: https://code.visualstudio.com/docs/getstarted/keybindings#_editorwindow-management

[tgeorgeux]

Firefox uses Ctrl+Page Up(left)/Page Down(right) or Cmd+Option+Left/Right. Chrome uses Ctrl+Tab, and Ctrl+Shift+Tab.

I think Ctrl Shift ./, is a great choice! It gives the user arrows to denote directions and mirrows the Ctrl Shift that advances tabs. @qntnrbns