Update graphical interface with new branding

[tturocy]

This updates the graphical interface to incorporate our new logo:

• Updates the MacOS .icns file for the app
• Refines the splash-screen experience to be a bit more modern
• Updates and simplifies the "About" dialog

These have been tested so far on MacOS - we need to review these on Linux and MSW. Specifically we need to see whether there's any adjustments to the MSW installer that can/should be made to incorporate the branding.

[tturocy]

@rahulsavani please try on Linux; @edwardchalstrey1 if we can manage to scare up a Windows machine somewhere to try this on it would be useful (there should be an installer built as an artefact on the GitHub action on the branch).

[edwardchalstrey1]

@rahulsavani please try on Linux; @edwardchalstrey1 if we can manage to scare up a Windows machine somewhere to try this on it would be useful (there should be an installer built as an artefact on the GitHub action on the branch).

Frustratingly though I did manage to send the app to a colleague with a Windows laptop, the Turing's IT settings meant it could not be opened without admin privileges - I don't currently have a credited Azure subscription or a Windows boot image for a cloud or local VM, so perhaps your son's laptop is the best bet for now @tturocy :)

Or if not I can ask IT to let me borrow a machine perhaps, doesn't look like anyone is in today though I could try next week