Change the symbol-component initialisation

[mattesmohr]

Change or enhance the initialisation of the symbol-component so the user can use a custom path for a symbol asset or the system symbols, wich come with the package.

// the custom path needs to be set first
Symbol(custom: "house-fill")

Symbol(system: "house-fill")

[mattesmohr]

New symbols were introduced with #119, like:

Symbol(system: "chart.pie")
Symbol(system: "clock")
Symbol(system: "envelope")
Symbol(system: "file")
Symbol(system: "folder")
Symbol(system: "house")
Symbol(system: "lock")
Symbol(system: "person")
Symbol(system: "photo")
Symbol(system: "text.aligncenter")
Symbol(system: "text.alignjustify")
Symbol(system: "text.alignleft")
Symbol(system: "text.alignright")

But the initialisation for customs still isn't implemented yet.