[Feature] Persistent scrollbar offset state

[Wdestroier]

In most widgets the parent passes the state to the child, but TrinaGrid creates the state and invokes a callback the parent can pass in the constructor to receive the state. If the parent widget rebuilds, then TrinaGrid may lose state (in this case the scrollbars state). Updating the scrollbar position after TrinaGrid invokes the callback makes the screen look glitchy, because it renders incorrect state for a few milliseconds until the next frame. To avoid a large refactoring which is probably undesired and solve this issue, I propose accepting an initialVerticalScrollOffset and another initialHorizontalScrollOffset parameter in the TrinaGrid constructor (Wdestroier@43e9fbe, Wdestroier@e409db9).