feat: Add Essentials Toolbar and hide Accordion Header in compact mode

- Added the Essentials Toolbar to the Navigator Toolbox
- Removed the Essentials Toolbar from ZenStartup and created it in browser-places.js
- Updated the zen-styles/zen-sidebar.css file to remove the display of the Menubar
- Updated the navigator-toolbox.inc.xhtml file to add the Essentials Toolbar and hide the Accordion Header in compact mode
- Updated the indicator-private-browsing.svg file to include the private browsing indicator icon

src/toolkit/themes/shared/icons/indicator-private-browsing.svg

@@ -0,0 +1 @@
+<?xml version="1.0" encoding="UTF-8"?><svg id="a" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20"><defs><style>.b{fill:#8000d7;}.b,.c{stroke-width:0px;}.c{fill:#fff;}</style></defs><circle class="b" cx="10" cy="10" r="9.5"/><path class="c" d="M12.7711,11.1656c-1.1727,0-2.1528.798-2.4543,1.876h-.634c-.3015-1.078-1.2816-1.876-2.4543-1.876-1.4113,0-2.5595,1.1492-2.5595,2.5606s1.1482,2.5596,2.5595,2.5596c1.1727,0,2.1528-.7973,2.4543-1.875h.634c.3015,1.0777,1.2816,1.875,2.4543,1.875,1.4114,0,2.5606-1.1483,2.5606-2.5596s-1.1492-2.5606-2.5606-2.5606ZM7.2284,14.9166c-.6568,0-1.1913-.5334-1.1913-1.1903s.5345-1.1914,1.1913-1.1914c.6564,0,1.1909.5345,1.1909,1.1914s-.5345,1.1903-1.1909,1.1903ZM12.7711,14.9166c-.6569,0-1.1904-.5334-1.1904-1.1903s.5335-1.1914,1.1904-1.1914,1.1914.5345,1.1914,1.1914-.5345,1.1903-1.1914,1.1903ZM16.3154,8.9273h-1.4463l-.6548-4.6247c-.0298-.2092-.1542-.3932-.3382-.498-.184-.1043-.407-.1198-.6003-.038l-3.276,1.3538-3.2761-1.3538c-.1953-.0817-.4178-.0668-.6003.038-.184.1049-.3084.2889-.3382.498l-.6543,4.6247h-1.4468c-.3777,0-.6841.3063-.6841.6841s.3063.6841.6841.6841h12.6313c.3783,0,.6846-.3063.6846-.6841s-.3063-.6841-.6846-.6841ZM7.0167,5.3681l2.722,1.1245c.1665.0699.3557.0699.5222,0l2.722-1.1245.5042,3.5592h-6.9741l.5038-3.5592Z"/></svg>