Emit and handle (custom) `-register-changed` notification This commit adds new (custom) asynchronous notification about register value being changed. Standard GDB does not notify MI clients about register value being changed when debugging (for example, by CLI command `set $rax = 1` or via Python's `Value.assign()`). This caused libgdb's register value cache being out of sync. In the past, this was partially worked around by manually emiting the notification on `GDBRegisterWithValue` APIs, but this did not (and could not) handle the case register was changed from GDB command line. To solve this problem, this commit installs a custom Python event handler that emits new GDB/MI notification - `-register-changed` - whenever a register changes after debugee is stopped. This has been enabled by upstream GDB commit 4825fd "gdb/python: implement support for sending custom MI async notifications" On libgdbs side, complete inferior state is invalidated. In theory, one could carefully invalidate only the changed `GDBRegisterWithValue` but in certain cases this could also change the backtrace (for example, if one updates stack pointer) or position in code. So it seems safer to just invalidate everything.

Load libbdbs-specific Python support code on startup This commit adds support for loading libgdbs-specific (GDB/MI) support code into GDB. This is a preparation for adding custom notifications and commands that are specific to libgdbs.

Compatibility: add portlib method returning directory with test programs ...as it cannot be easily determined in a dialect-agnostic way.

Do not wait for MI response when using `GDBDebugger >> #pythonImport:` ...in order to speed up startup.

Handle leading `-` when mapping async notification to class name GDB commit 825fd2d "gdb/python: implement support for sending custom MI async notifications" introduced support for user-defined asynchronous notifications. Such notification can (in fact, should!) start with hyphen (`-`) and may contain sequence of hyphens. This commit fixes `GDBAsyncEvent class >> #eventClassForType:` to handle such cases.

Fix `GDBMIDebugger` after rename of `GDBStXUnixProcess` to `GDBUnixProcess` ...in commit d1422e1ee.

Rename `GDBRegisterValueChangedEvent` to `GDBRegisterChangedEvent` ...to make name consistent with `GDBMemoryChangedEvent`.

Add inspector tabs showing breakpoints, inferiors, ... ...threads, backtrace and registers, just like in Pharo port (see commit b333314e)

Add MI parser test This commit add test to parse real-world frament which failed to Pharo properly at some point. It is encoded here as bytearray to make sure all the characters are preserved exactly as they were.

Pharo port: add initial set of GT inspector extensions