Browser view refactor

[pfrazee]

In an effort to solve the freezing issue (#1333) I've decided to refactor Beaker to use the BrowserView API rather than <webview>. This was a planned update already, but the freezing issue forced me to prioritize it. There will be other advantages to usability; most notably, devtools will be able to dock!

Implementing this change is a pretty big undertaking. Because I can't render shell-window HTML on top of a BrowserView, I have to reimplement all modals and menus as sub-windows that float over the shell window. Given the magnitude of that change, I decided now would be a good time to reimplement basically all of the shell-window UI code (which was pretty crufty). That includes a switch from yo-yo to LitElement, which I've been getting great results from lately.

Todo list:

• Migrate to BrowserViews
• Shell-window UI (tabs, navbar)
• The window menu
• Track view load state
• Pinned tabs
• Tab dragging
• Tab context menu
• Location bar w/auto-complete
• Location bar context menu
• Status bar
• Browser menu
• Bookmark menu
• Page menu
• Seeding menu
• Donate menu
• Site-info menu
• Sync folder menu
• Browser update available menu
• Watchlist indicator button
• Downloads UI
• In-page context menu
• Permission prompts UI
• Create dat modal
• Fork dat modal
• Select dat modals
• Basic auth modal
• Prompt modal
• Live reloading
• Zoom control
• Inpage find
• Retained scroll y on refresh
• Fullscreen controls
• Tab media paused/playing/muted
• Dat / https alternative detection
• Markdown renderer
• JSON renderer
• Swipe-to-navigate (removed)
• Dat timeout detection
• window.open
• Keyboard shortcuts
• Focus-location command
• Focus the location bar on "new tab"
• Location bar autocomplete results need to be clickable
• Replace titles in history if they change soon after navigation (updateHistory())
• Site load errors
• On page.url change, main is notified so that it can reconstruct the window-menu. (shell-window:set-current-location)
• Tab-state snapshots
• Make sure "onbeforeunload" is working
• Restore tests
• Final QA: Overall visuals
• Final QA: Double check all old behaviors were carried over
• Final QA: Windows
• Final QA: Linux
• Final QA: Test the auto-updater with a live system

Issues:

• There appears to be an RPC stream leak (?)
• View background color is off sometimes
• Keyboard shortcuts stop working after focusing browserview
• Subwindows should never handle window-menu events or keyboard shortcut events
• Scrollbar shows up in the statusbar UI sometimes
• The location bar "replace selected text" doesn't always work
• Location bar should show URL that's being loaded, but go back to the old URL if canceled

[pfrazee]

Frustratingly, BrowserView reduces but doesn't eliminate the freezing issue. Webviews will freeze on any site, whereas BrowserView will only freeze on custom protocols.

EDIT: No worries, got a fix in the pipeline with the Electron team.

[pfrazee]

Almost there! Minus the vendored LitElement code, this PR is currently +7891/-710 (!)

I'm going to merge without removing some dead code so that I can also merge into the blue branch (which has modifications to some of the dead code). After merging, I'll make another PR that removes the dead code.

[pfrazee]

👍 This PR is ready to go. Merging now!

This won't fix the freezing issue. That's now being addressed with electron/electron#17221. We'll wait for that fix to land in electron to release a new Beaker with this PR.

Any folks building from source, feel free to give the new master a try and let me know how this refactor works for you. Hopefully there aren't too many new bugs or regressions.