Add splash screen, window icon and other GUI improvements #351

Job	Run time
Build MDANSE and run tests on ubuntu-20.04	3m 33s
Build MDANSE and run tests on ubuntu-20.04	3m 25s
Build MDANSE and run tests on ubuntu-20.04	3m 20s
Build MDANSE and run tests on ubuntu-20.04	3m 51s
Build MDANSE and run tests on windows-2019	5m 12s
Build MDANSE and run tests on windows-2019	4m 4s
Build MDANSE and run tests on windows-2019	4m 15s
Build MDANSE and run tests on windows-2019	5m 52s
Build MDANSE and run tests on macOS-11	6m 2s
Build MDANSE and run tests on macOS-11	5m 43s
Build MDANSE and run tests on macOS-11	6m 8s
Build MDANSE and run tests on macOS-11	6m 27s
	57m 52s

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