Some uia v3 fixes

Most interesting commits are 'Fixes issues with WPF ComboBox' and 'Fixes #186: ...'.

I don't work with WinForms now, so I lack a bit of motivation to fix problems there - especially for ComboBox. It is not so hard, and in fact I have changes for WinForms as well, but tests for WF are still failing for some reason not related to these changes in any way. Namely, this one:
```
TestStack.White.Mappings.ControlDictionaryException
Multiple TestControls found for ControlType=list item and FrameworkId: - TestStack.White.UIItems.ListBoxItems.Win32ListItem, TestStack.White.UIItems.ListBoxItems.Win32ListItem, TestStack.White.UIItems.ListBoxItems.WPFListItem, TestStack.White.UIItems.ListBoxItems.WPFListItem, TestStack.White.UIItems.ListBoxItems.WPFListItem
```

Similar issues with some other tests as well. If someone will find time to look at them - ComboBoxTests.cs, try to execute WinForms tests. Wpf should be passing, WinForms - fail.