Move labeling UI into Inspector sidebar pane

[erikrose]

Once https://bugzilla.mozilla.org/show_bug.cgi?id=1398734 is done, we'll be able to move our labeling UI from its current home in a separate Fathom devtools panel to a subpane of the Inspector panel. This will greatly speed the labeling experience, relieving people of pogo-sticking between the two panes.

[erikrose]

It's done! Now we can work on this.

[erikrose]

We might want to keep parts of the devpane on their own panel for an overview, but, at the simplest, this could look like this:

async function createPanel() {
-    const extensionPanel = await browser.devtools.panels.create(
-      'Fathom',
-        '/icons/icon.svg',
-        '/pages/devtoolsPanel.html');
-    extensionPanel.onShown.addListener(panelShowed);
-    extensionPanel.onHidden.addListener(panelHid);
+  const sidebar = await browser.devtools.panels.elements.createSidebarPane("Fathom");
+  sidebar.setPage('/pages/devtoolsPanel.html');
+  sidebar.onShown.addListener(panelShowed);
+  sidebar.onHidden.addListener(panelHid);
}

See rpl's branch at https://github.com/erikrose/fathom-fox/compare/master...rpl:experimental%2Finspector-sidebar-panel?diff=unified.