Rewrite to new iTasks-electron interface without message passing

Main tasks to be done:

• Search bar
• Search from the context menu ("Search with this...")
• Distinct column values (closes #35 (closed))
• Menu items should disable/enable depending on the currently focused window
• Add/delete column (closes #40 (closed))
• Warning when quitting with unsaved changes
• Check that multiple open sheets works fine; add Ctrl-W to just close one sheet (closes #34 (closed))
• Add interface to create a new sheet (closes #20 (closed))
• Settings dialog
• The welcome window should close as soon as you create a new sheet or open one
• Check that all the entries in the menu work
• Remove dead code
• Update screenshot in readme
• Bump version in package.json

Also closes #36 (closed); closes #37 (closed); closes #38 (closed).

This is currently blocked on:

• https://gitlab.science.ru.nl/clean-compiler-and-rts/compiler/-/issues/79
• https://gitlab.science.ru.nl/clean-and-itasks/clean-libraries/-/merge_requests/12