Add on_changed setting hook

[hugsy]

Description

This PR fixes some issues in the setting loading process resulting in the default value always being taken for gef-extras scripts (as underlined by hugsy/gef-extras#107

GefSetting now offers the possibility to trigger a callback, on_changed which behaves just like on_write but only if the value was changed.

Checklist

• My code follows the code style of this project.
• My change includes a change to the documentation, if required.
• If my change adds new code, adequate tests have been added.
• I have read and agree to the CONTRIBUTING document.

[github-actions]

🤖 Coverage update for 071046a 🟢

	Old	New
Commit	13af366	071046a
Score	71.6374%	71.6374% (0)

[hugsy]

As an example, gef.extras_plugin_dir was added as on_changed, allowing extras plugins to be loaded at anytime:

$ gdb -q -nx
(gdb) source ~/code/gef/gef.py
GEF for linux ready, type `gef' to start, `gef config' to configure
89 commands loaded and 5 functions added for GDB 12.1 in 0.00ms using Python engine 3.10
gef➤  gef config gef.debug True
gef➤  gef config gef.extra_plugins_dir ~/code/gef-extras/scripts                                                                             [=] in __invoke_changed_hooks("gef.extra_plugins_dir"), setting.value='' -> new_value='~/code/gef-extras/scripts', changing=True
[=] Loading extra plugins from directory=PosixPath('/home/hugsy/code/gef-extras/scripts')
[=] Loading '/home/hugsy/code/gef-extras/scripts/assemble.py'
[=] Loading '/home/hugsy/code/gef-extras/scripts/bincompare.py'
[=] Loading '/home/hugsy/code/gef-extras/scripts/bytearray.py'
[=] Loading '/home/hugsy/code/gef-extras/scripts/capstone.py'                                                                                                                                                                                                                     [=] Loading '/home/hugsy/code/gef-extras/scripts/ftrace.py'
[=] Loading '/home/hugsy/code/gef-extras/scripts/ida_interact.py'                                                                            [=] Loading '/home/hugsy/code/gef-extras/scripts/peekpointers.py'                                                    
[...]
[=] Loading '/home/hugsy/code/gef-extras/scripts/syscall_args/syscall-tables/X86_64.py'
[+] 42 extra commands added in 0.39 seconds

[github-actions]

🤖 Coverage update for fddba95 🔴

	Old	New
Commit	13af366	fddba95
Score	71.6374%	71.5933% (-0.0441)

[github-actions]

🤖 Coverage update for fddba95 🟢

	Old	New
Commit	13af366	fddba95
Score	71.6374%	71.6374% (0)

[therealdreg]

LGFM! @hugsy