Let a "Breakpoints" window (by default opened by Debug>Windows>Breakpoints [ctrl+B, D]) serve as an example. Basically I select few breakpoints in it and I would like to know in my add-in which elements in this window are selected. I am aware that I can get collection of breakpoints in project but I would like to know what elements are selected in "Breakpoints" window.
"Is it possible to get selected items in window or even access its content at all?"
Also I am not sure whenever or not should I post a separate question for this but is there actually a way to capture user activity in IDE like for example capturing an event when user sets (adds) a breakpoint?
Originally I also asked if is it possible to achieve certain things in Visual Studio Express Edition. But this part is irrevelant.
(after reading jessehouwing's answer)
I guess it is not possible using an Add-ins. Use VSPackages isntead. Also Add-ins are deprecated as of Visual Studio 2013 version.
As mentioned in my comments, what you're trying to accomplish is explicitly prohibited in the Visual Studio Express edition and is a violation of it's license. To extend the product, you need to have at least Visual Studio Professional Edition. many of the extensibility points will actively refuse any communication with 3rd party add-ins.
Almost all the things you're asking are possible using Visual Studio Extensibility once you've installed the professional edition. Products like OzCode show that almost everything is possible. Remember that most features inside visual studio are themselves extensions of the product.
Your question, indeed a whole list of questions, is indeed not the way to ask something on StackOverflow. I can give you some pointers to the documentation, which you've probably already found, and maybe to some open source products that themselves extend parts of Visual Studio that can serve as examples, but from there you'll have to piece something together until you're able to ask more specific questions.
Projects that extend the debugger that might serve as an example:
But there is no easy answer to your question that fits inside this window. I'd suggest you use a tool like Reflector to look at how Microsoft accomplishes certain things (most of Visual Studio Extensibility is written in .NET anyways) and to look at open source projects that extend visual studio behavior. There are quite a few out there on Codeplex.
I'm not entirely sure what you're trying to accomplish and how it's different from the Breakpoints features inside Visual Studio Professional and up.
I suggest you ask your question in the Visual Studio Extensibility forums over on MSDN, which is in a collaborative forum format, instead of a Q&A format, allowing people to answer your question bit by bit.