Alpha blending support for text windows and dialog options

* Custom text windows now support alpha blending;
* Dialog options overlay supports alpha blending;
* Added DialogOptionsRenderingInfo.HasAlphaChannel property.

Text window and dialog options alpha blending is only active in games made with AGS 3.3.0 or higher. This is done for backward-compatibility: the older games must look as they did before.
.gitignore: fixed version.aps location