Support grouping of registers

In GDB, [registers ](https://sourceware.org/gdb/current/onlinedocs/gdb.html/Registers.html) can be categorized by groups.
Groups and their registers can be announced as part of the target description XML, see bottom of: https://sourceware.org/gdb/current/onlinedocs/gdb.html/Target-Description-Format.html There are predefined groups, but also custom grouping is handled.

Depending on the number of registers to display, it can become tedious to navigate through them in the Variables window.
I'd propose to group registers and display a tree accordingly to make navigation more efficient.