[electron] improve application icon

[vince-fugnitto]

Description
The current icon for the application does not have a good visibility (especially on dark themes).
I propose we instead take some time to create a better icon.

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Current = Left | Example Proposal = Right

Additional Info
Taken on macOS dark mojave.

[stale]

This contribution has been automatically marked as stale due to inactivity, and it will be closed if no further activity occurs. Thank you for contributing to Theia!

[vince-fugnitto]

Unfortunately, the icon is still an issue for dark themes on any operating system.
Some work should be done to make the icon more visible for the packaged electron app.

[stale]

This contribution has been automatically marked as stale due to inactivity, and it will be closed if no further activity occurs. Thank you for contributing to Theia!