Refactor access modifiers in base classes

[jaybyrd42]

Add possibility to override OnMouseDown, OnMouseUp and other related event handlers and reuse logic from those overriden methods in Classes derived from LiveChartsCore.Chart by changing InvokePointerDown, InvokePointerMove, InvokePointerUp, InvokePointerLeft... access modifiers to protected internal

[jaybyrd42]

Possibly refactor all internal modifers to protected internal.

Having internal access modifiers when using this library as Nuget package hinders modification