[HxGrid] Placeholder rendering performance optimization

[hakenr]

Suggestion by @crdo:

I found what I think is a nice spot for improving the performance of rendering those placeholders.
Jirka everywhere added <span class="placeholder-glow"><span class="paceholder"></span></span> and it seems that he puts that placeholder as a Blazor component into each column.
In reality, however, only the inner part, i.e., <span class="paceholder"></span> is needed, and the class determining the animation of the placeholder can be applied just once somewhere at the root of the table. As it is now, it's being repeated in each cell over and over again.

[hakenr]

Iteration 1 - switched Blazor components to HTML elements (4.5.0-pre1)