Support global and file static variables

The CDT GDB adapter currently only displays local variables and registers in the Variables view.
However, you also may want to see global variables and/or file-static variables (see [here](https://sourceware.org/gdb/current/onlinedocs/gdb.html/Variables.html))

GDB supports enumeration and access through https://sourceware.org/gdb/current/onlinedocs/gdb.html/GDB_002fMI-Symbol-Query.html and https://sourceware.org/gdb/current/onlinedocs/gdb.html/GDB_002fMI-Variable-Objects.html#GDB_002fMI-Variable-Objects

We should add support for global/file-static variables. You currently can only inspect them from the Watch or the Debug Console if you know a certain variable name, or through GDB CLI commands.

Potential caveats
* Depending on the program, you may have a significant number of such variables. Showing them as a flat list may become expensive on Variable view updates for embedded targets due to the target accesses.
* File-statics could be always shown or only when they are in scope, i.e. PC is at code from a specific file.

Hence, we need to keep an eye on performance during work on this feature. And may need a setting as a fail-safe to turn this off.