Status Bar should be optional and toggled from menu or similar

[mbektas]

There is no way to hide the status bar to save on screen real estate. A menu item can be added to toggle it.

[jasongrout]

Sounds good. I think it makes sense to have a menu entry in the view menu to toggle this status.

I can imagine that a good way to tackle this is to

1. Make a new command to do this, similar to the "Show Left Sidebar" command at

   jupyterlab/packages/application-extension/src/index.tsx

   Lines 463 to 478 in a496aad

   	app.commands.addCommand(command, {
   	label: args => 'Show Left Sidebar',
   	execute: () => {
   	if (app.shell.leftCollapsed) {
   	app.shell.expandLeft();
   	} else {
   	app.shell.collapseLeft();
   	if (app.shell.currentWidget) {
   	app.shell.activateById(app.shell.currentWidget.id);
   	}
   	}
   	},
   	isToggled: () => !app.shell.leftCollapsed,
   	isVisible: () => !app.shell.isEmpty('left')
   	});
   	palette.addItem({ command, category });

2. Add it to the menu, like the notebook adds items to the view menu in

   jupyterlab/packages/notebook-extension/src/index.ts

   Line 1975 in 930420a

   mainMenu.viewMenu.addGroup(collapseGroup, 10);

3. Make it a setting that is saved for the status bar extension (a little more complicated, but basically adding a new schema file for the status bar, and then putting logic into the command to change the setting when the status bar is toggled).

[jasongrout]

Tagging as a good first issue - to do everything above is probably an advanced good first issue.