Introduce `GDBPortlib` to facilitate porting to other dialects This commit introduces `GDBPortlib` in order to allow port to other Smalltalk dialects, e.g, Pharo. Dialect specific code should go there. For now, this includes creation of pipes and PTYs and spawning processes. Since now we use `GDBPortlib`s to encapsulate dialect-specific code, (what used to be) `GDBStXSimpleProcess` and `GDBStXUnixProcess` are no longer Smalltalk/X specific. Therefore they have been renamed to `GDBSimpleProcess` and `GDBUnixProcess`

Compatibility: do not use `Delay waitForMilliseconds: x` ...and use `(Delay forMilliseconds: x) wait` as the latter is more portable.

Compatibility: remove dependency on `UserPreferences` ...as this is very Smalltalk/X specific class. Instead, keep user-configured GDB in class variable in class `GDBProcess` and let dialect-specific code to set as required.

Compatibility: handle different implementations of announcements This commit refactors the code using and extending annoucements to handle different implementations present in Smalltalk/X and Pharo.

Compatibility: use `#includes:` rather than `#includesIdentical:` ...as the latter is not present in (at least) Pharo.

Compatibility: do not use `#nextPutLine:` ...as it does not exist in (at least) Pharo. Also, `#flush` the output stream once the command is written as Pharo's stream are buffered.

Compatibility: do not use `#findFirst:ifNone:` ...as it does not exists in (at least) Pharo.

Compatibility: refactor finalization of debuggers and variable objects ...to make it compatible with (at least) Pharo. It also changes the code so that all GDB-related finalization uses one single registry (instead of having variable object finalization registry per debugger object).

Compatibility: do not use `Process >> addExitAction:` ...since it does not exists in (at least) Pharo. Instead, use `#ensure:` in process's start block.

Compatibility: use `#cull:` instead of `#valueWithOptionalArgument:` ...as the latter does not exists in (at least) Pharo.