Added scrolling for dropdown panel

[Tharylia]

This PR adds scrolling to the dropdown panel.

In the current implementation the dropdown panel shows all items at the same time causing it to go offscreen in some cases.

The new implementation adds the field "PanelHeight" to the dropdown control. The field defines to height which the dropdown panel should occupy. If more items are added as can fit in the panel, a scrollbar will be shown. In the default configuration, nothing changes.

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