Suggestion to Add Adjustable Position Feature for ComboBox Drop-down Arrow（建议添加 ComboBox 下拉箭头位置可调整功能）

[dotnet9]

Currently, the drop-down arrow of the ComboBox in the Semi.Avalonia control library is fixed on the right side. Under certain specific interface layout and design requirements, this fixed position may not meet diverse design needs. It is hoped that a function can be added to allow developers to adjust the position of the ComboBox drop-down arrow to the bottom according to the actual situation. This can greatly improve the applicability and flexibility of the control in different scenarios, thereby better meeting various user interface design requirements.

当前 Semi.Avalonia 控件库中的 ComboBox 下拉箭头固定在右侧，在某些特定的界面布局和设计需求下，这种固定位置可能无法满足多样化的设计要求。希望能够添加一个功能，允许开发者根据实际情况将 ComboBox 下拉箭头的位置调整为下方，这样可以极大地提升该控件在不同场景下的适用性与灵活性，从而更好地满足各种用户界面设计需求。