Plugins: Allow plugins to add Layout and Block methods

Similar to how a variety of class methods (example: pageMethods) can be added using plugins, add support for the recent Layout and Block classes:

• layoutsMethods
• layoutMethods
• layoutcolumnMethods
• blocksMethods
• blockMethods

Example use cases are overwriting the span() method, or adding functionality such as a hasBlocks() method, as suggested in https://kirby.nolt.io/200.

This would also be really handy for https://github.com/sylvainjule/kirby-footnotes