make TriggerBase to "public"

Many times we need more special triggers
Like what
1. Judgment of greater than or equal to and not equal to conditions
2. There is an OR relationship between multiple conditions
3. Mix between DataTrigger and Trigger

Therefore, can we change the constructor, GetCurrentState, and TriggerConditions of TriggerBase to Public, giving more possibilities without affecting the original code and writing

from [Bing Microsoft Translator]
[7472](https://github.com/dotnet/wpf/pull/7472)

我们很多时候需要更特别的Trigger
比如
1. 大于等于、不等于等条件判定
2. 多个条件之间是"或"关系
3. DataTrigger和Trigger之间混用

因此我们是不是可以把TriggerBase的构造函数、GetCurrentState以及TriggerConditions属性等更改为Public，在不影响原有代码和写法的同时给予更多的可能性。