This PR contains a number of smaller user-facing changes/adjustments that are intended to clarify or make more explicit the functionality that was hitherto broadly referred to within the application as "sharing". Because user-expectations pertaining to the concept of "sharing" have been observed to differ wildly between users (e.g., sharing as "publication", vs. sharing as "collaboration", vs. sharing as "sending" to another person/device one-time-only, and so), and because Gridsync's "sharing" feature in particular is considerably more narrow and different than these (consisting, more precisely, in the transfer of Tahoe-LAFS storage fURLs and/or capabilities over ephemeral magic-wormhole channels), the functionality being offered to the user through the language of the application rarely matched user-expectations. Accordingly, this PR attempts to ameliorate this disjunction by replacing broader instances of "sharing" within the interface with more precise language, concepts, and icons that (hopefully) more closely match or reflect the underlying functionality. More specifically, this PR:

• Removes the dual-use (and thus ambiguous) "Share" button from the MainWindow toolbar altogether
• Updates the "Enter Code" button with an "Invite" button which, when clicked, presents the two more explicit/precise menu actions of "Enter Invite Code" or "Create Invite Code"
• Removes the top-level "Share" action from the right-click/context menu, replacing it with a "Sync with device" -> "Create Invite Code" menu/action hierarchy (which provides the same functionality as before)
• Updates the "user" overlay icon for (what were previously known as) "shared" folders with a "laptop"/"device" and removes references to "shared" and "person" in the underlying tooltip text (in order to reflect or encompass the seemingly more likely situation of a user syncing a folder with another device that they own, as opposed to "sharing" with another "person" -- which can mean many things)

In addition, other, non-sharing-related UI adjustments have also been made to better balance or make more consistent some adjacent actions. For example, the "Preferences" action has moved to the systray menu (since this is arguably not a "primary" action), the "Export Recovery Key" action has been removed from the systray menu (since it was already available via the MainWindow toolbar), the "History" toolbar button/toggle has been moved to the right-hand side of the grid-selector/combobox (to take the place of the newly-removed "Share" button), and tooltip text has been updated throughout to be more more consistent with regard to capitalization.