In accordance with #92, this PR adds a per-grid "history view", currently accessible via toolbar button:

In addition, a new bottom "status panel" has been added beneath it, offering a space into which more grid-specific information can be presented (which otherwise would require the folders-view to be visible). Currently, this provides an "overall" status indicator on the left (switching between "Connecting...", "Syncing", and "Up to date"), a conditionally-present Tor icon (displayed only if connected to the grid over Tor), and a "globe" icon with grid status information in the tooltip (i.e., nodes connected, space available) as gathered from the Tahoe-LAFS WUI "welcome page". This panel can stand to be improved significantly with additional functionality and information in the future (e.g., by making the icons clickable buttons for performing various actions manually or by including additional indicators for various grid-status events) but is useful enough as a replacement for the folders-view's statusbar to warrant being included now.

In terms of per-file operations on items within the history view, currently only "open file" and "open enclosing folder" are available (via context menu and/or triple-dot-button); this can be expanded with additional actions later (e.g., for sharing individual files, restoring deleted files, browsing older versions, etc.).

Lastly, this PR includes a few misc. macOS-specific tweaks (like unifying titles and toolbars, adjusting font point-sizes, and enabling support for high-DPI "retina" displays on ".app" bundles) which, in retrospect, probably should have been included in a separate PR (but which, nevertheless, should hopefully improve visual appearance on Macs). :)