Enhancement: Special/Custom buttons #9

There's actually a lot of things that a user might want to do depending on the usecase with custom functionality.

Idea for that: Define a custom data structure for a custom key, add a list of that as configuration parameter ("customKeys") and make it possible to reference these from a layout. That data structure could include text, an optional symbol (to be shown additionally or instead of the text), sizing constraints and a callback for whatever the button should do.

One example in my use case: I have some numbers in my app that a user can edit, which are likely negative. It's handy to have a "+-" key for that in a NumericLayout which toggles the sign of the current value that's being edited.