Support viewing registers by group

[xusheng6]

Registers can be grouped into several groups, e.g., general purpose, floating point, etc. We should read these info from the backend and offer to view registers by group.

We should probably default to show full-width general purpose registers, which is the most common set of registers people wish to see. Related to #221.

Currently, LLDB backend supports reading the register group info through its C++ API. Things are complicated for DbgEng. Although it internally understands different register groups, and function like IDebugRegisters2::OutputRegisters2 supports printing a particular set of registers, there seems to be no API to read the group attribution of a particular register. In other words, we may not be able to read the register group information from the DbgEng backend.

We may hard-code the register group info into the debugger code to avoid doing more work. Both LLDB/DbgEng register group info are pretty stable and unlikely to change.