Consolidate localization behavior and init to a coreLocalizer function and singleton
Explicitly support `en-US` locale
Rename coreData to coreFileFetcher and export a singleton rather than using a property of coreContext
Add `apiConnections` property of coreContext to simplify adding a source switcher
Replace some init functions with re-callable, promise-supporting `ensureLoaded` functions
Make coreContext itself load the UI if a container has been specified at init time
Fix code tests
Generalize tooltip into popover control
Use the same popover control for tooltip as the preset browser and tools list popovers
Smartly position the preset browser popover and menu bar tooltips to stay fully onscreen
Position most tooltips closer to their controls
Fix small gap that could appear between a tooltip and its arrow
Allow wider toolbar tooltips
