[gdbplugin] GDB backend reimplemented using GDB/MI

Currently the GDB parser uses regular expression on the output produced
by the user-oriented GDB console to capture needed information.
While this approach works fine for english versions, it is not robust
neither to different localisations nor to reworded outputs in newer
versions.

This patch provides a reimplementation of the GDB backend using the
GDB Machine Interface
(https://sourceware.org/gdb/onlinedocs/gdb/GDB_002fMI.html), which it is
invariant to localizations and provides a stable interface. As a side
benefit, other debuggers which implement GDB/MI can be used too.

The patch has been tested with gdb 12.1 and lldb-mi (based on gdb 7.4,
lldb 14.0.0).

Functionality changes:

- Action "continue" does not stop at start.
- Support for pending breakpoints (useful for debugging dynamically
  loaded libraries).
- The variable explorer shows member of "this" object in a separate
  variable scope.

Possible improvements to be done in the future:

- Loading large variable scopes can be very slow in gdb (it is faster in
  lldb). May be using lazy loading for nested variables could mitigate
  it.
- The nested variable parser could be improved to detect several complex
  nested variables.
- Although remote targets can be used, more testing is needed.
- Implementing breakpoint table persistence could be nice for repeated
  debugging.