Improve the default look of showAboutWindow() on Linux and Windows

[mquevill]

Since defaults don't exist for Linux in the base Electron package (electron/electron#18918), I figure that showAboutWindow() could set some of these defaults and then use the standard About dialog. Until the defaults are standardized by the Electron (electron/electron#23851), this would be a way to get consistency across Windows and Linux.

The documentation would need to be updated to reflect these changes, which I can add after the changes are finalized.

Example using Caprine:
About window on main branch:

With this PR:

Adding copyright and website to Caprine's about menu:

[Note: This does not fix the icon getting messed up when packaged into a .deb file.]

Perhaps the defaults set here should be marked with a TODO comment while waiting for Electron to set defaults?

[sindresorhus]

Perhaps the defaults set here should be marked with a TODO comment while waiting for Electron to set defaults?

👍 Always good to add TODOs for future possible improvements (and include links to relevant issues)

[sindresorhus]

I'm ok with this. Feel free to update the docs (and index.d.ts)