Customize symbols instead of layouts

[herrbrixter]

**Is your feature request related to a problem?
I'd like to customize the symbols on the keyboard. I prefer to use the symbols layer because they are my most used symbols and should have higher priority. I use layout layer for occassionally used symbols and therefore have lower priority.

Describe the solution you'd like
Ability to change the symbols at the symbol level

Use case
I seldom use underscore (_) and curly braces { }. I would like to use slash / instead. The slash is not in the symbols layer outside of URL fields but I use slash more often than underscores.

Describe alternatives you've considered (if any)
I have to use the layout layer but it has a lower priority than the symbol layer. I don't want to mix high priority and low priority symbols in the layout layer.

[herrbrixter]

I also don't use the inverted punctuations like ¡ and ¿.

As a foreign language speaker, I'd rather replace unused symbols with foreign characters like ä, ß, ñ which I use everyday.