testing/signal-desktop: upgrade to 7.35.1

https://github.com/signalapp/Signal-Desktop/releases/tag/v7.35.1

• Adds README
• Renames _check_depends to _update_depends and makes it so that it updates the relevant variables using sed
• builddir is set as $srcdir so that dependencies are built under ./src rather than ./src/Signal-Desktop-$pkgver. This implicated src/Signal-Desktop-$pkgver being treated like its extra dependencies, thus -$pkgver is removed and relevant patches are modified to take that into account. This change fixes an issue where ringrtc would pickup node-modules from Signal-Desktop rather than use its own.