Make "new window" and "new tab" menu item labels consistent

There are a few menu items in the UI that let the user open a new window. They have inconsistent names:

File menu: "New Tab" (Ctrl + T), "New Private Window" (Ctrl + N) Hamburger menu: "New Window" (Ctrl + N) Context menu on links: "Open Link in New Tab", "Open Link in New Private Window"

We should make these names consistent, I think. Should it be "New Tab" and "New Window" or "New Private Tab" and "New Private Window"? All tabs and windows in Tor Browser are supposed to be private. :)

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