fix: can open IDE2 from an ino file

[kittaakos]

Motivation

Can open IDE2 from an ino file.

Change description

Use a fallback frontend app config when starting IDE2 from an ino file
(from Explorer, Finder, etc.) and the app config is not yet set.

Other information

See the Beta Testing Guide on how to verify the changes.

Preconditions:

• make sure the tester build is your default app to open ino files (On macOS, I had to wipe all previous IDE2 versions)

Steps to verify:

• download the tester build,
• start the IDE2 from its app icon (it starts),
• open more than one sketch and quit,
• start the IDE2 from its app icon (it starts, all previously opened (saved!) sketch windows open),
• quit IDE,
• open IDE2 by double clicking on an ino file (in Explorer, Finder, etc.) (it opens the sketch),
• quit IDE,
• start the IDE2 from a command line: for example /Applications/Arduino\ IDE.app/Contents/MacOS/Arduino\ IDE ~/Documents/Arduino/hello on macOS (it opens the hello sketch).

Closes #2209

Reviewer checklist

• PR addresses a single concern.
• The PR has no duplicates (please search among the Pull Requests before creating one)
• PR title and description are properly filled.
• Docs have been added / updated (for bug fixes / features)

[davegarthsimpson]

All good!