Desktop user experience overhaul

[SleepyLeslie]

This PR refactored a major portion of Grist Desktop:

• Dismissed use of symlinks (properly fixes Exporting and then opening a .grist file doesn't work correctly #4). See below for details. Closes Does not work if you disallow ~/Documents access #25.
• Implemented WindowManager.
• Implemented importing from the "file" menu (closes import on windows is dodgy #3, but needs more testing).
• Revamped the intricate pendingPathToOpen mechanism to make it more straightforward.
• Removed shouldQuit and handle single instance locks outside of GristApp.
• Moved all CLI argument handling code together, including the macOS -psn_ argument hack and file opening logic.
• Replaced preload.js with preload.ts, and set up electron IPC properly.
• Cleaned up the codebase and removed some now unused functions.

Filesystem changes

Grist Desktop used to rely on symlinks to comply with Grist Core's requirement that all documents be stored centrally in GRIST_DATA_DIR. This causes issues on Windows, and impacts the user experience since users cannot choose where to store their Grist documents.

Moreover, this contributed to a bad user experience as documents use docID as name, and GRIST_DATA_DIR defaults to the "Documents" folder. This means users will end up having a lot of <random string>.grist files in their "Documents" folder.

It has also led to complaints from macOS users, since they basically cannot use Grist Desktop without granting permissions to the "Documents" folder.

This PR enables Grist Desktop to open Grist documents anywhere on the local filesystem, without making symlinks. All modifications will directly write to the document.

Window manager

Grist Desktop now knows all documents it has open, and would bring the open document to you if you open it twice. It can keep track of window content changes (i.e. when a user navigates away from / into a document).

It must work together with this PR for grist-core gristlabs/grist-core#1099

[SleepyLeslie]

Tested to work on Linux, Windows 10 and Intel Macs; can open documents and symlinks to documents anywhere on the filesystem.

• macOS open-file events are handled correctly.
• Can use CLI argument to open file (Windows and Linux) correctly.
• Single instance lock is handled correctly.

Changes unrelated to the filesystem don't seem to break anything (good!)

Some comments:

• Can no longer create new document. This is because new documents don't have externalId by default. More changes to grist-core is required. This must be fixed. We can probably also support the "file -> new" menu properly.
• To prevent opening the same document twice, grist-desktop tracks open documents with a map from document IDs to Electron windows. If a user attempts to open an already open document, the corresponding window is raised. However, this mapping doesn't change when the user navigates to the home page or to another document, leading to weird behavior.
• After a document is deleted, it is still shown in the "All Documents" list. Attempting to open it will result in an unrecoverable error. It can be dismissed by clicking outside and going back to the home page, but this is bad UX. We should report an informative error message (rather than the dummy "error accessing document"), take the user back, then delete the entry from the database automatically.

[SleepyLeslie]

Now it is possible to create new documents using the in-app "Create Empty Document" button. However, this button will only work for the electron user. Should any user choose to use Grist Desktop as a self-hosted server, external visitors will now be unable to create new documents - they will get an error when clicking on the button.

[SleepyLeslie]

Implemented WindowManager to track open documents properly.

[paulfitz]

maybe no-longer-needed instead of something just to emphasize it isn't important anymore, we haven't just forgotten to put something secure here.