Support custom styles for LayoutGridResizerControl

[mpondo]

Summary
This change adds support for specifying a custom style to be used for the generated vertical and horizontal grid resizers.

Description of the Problem
Although it is possible to apply a default style to all instances of the LayoutGridResizerControl, our styling team had the desire to apply a custom style to only the vertical style. This was not currently possible, since there were no existing hooks for setting the style for this control.

Proposed Solution
This code adds properties to allow custom styles to be specified for either the vertical or horizontal LayoutGridResizerControl that is created. If the property is not set, the code will behave as before.

[Nfactor26]

It appears this change breaks existing style. After upgrading to 4.6.0 version or higher, I need to explicitly set GridSplitterVerticalStyle and GridSplitterHorizontalStyle on DockingManager.

Probably, we need to check for null before setting the style . My guess is unless GridSplitterVerticalStyle and GridSplitterHorizontalStyle are not set on DockingManager, we are setting style to null which doesn't pick the default style from theme.

splitter.Style = _model.Root?.Manager?.GridSplitterVerticalStyle;
splitter.Style = _model.Root?.Manager?.GridSplitterHorizontalStyle;