Simplify icon mapping code

• Breeze is the mandatory theme on macOS and Windows, so make it explicit
• setThemeName() is only supported on non macOS/Windows platforms
• Add a single iconMappings container for all Icons::Icon enums to theme mappings
• remove the KDE icons distinction – keep things simple and replace it with Oxygen and Breeze seperate mappings.
• simplify the convoluted setIconThemeNames() mapping logic