Signal for more generic function call interceptions

[ThiefMaster]

Recently we've got a bunch of PRs adding signals that are pretty much just providing a way to tamper with the arguments of certain function calls. And while we're open to accepting such PRs, I had the feeling that this was getting a bit messy, so I thought about providing a more standard way to handle this. While it would still require PRs to adjust a function call in order to provide this type of hook, it'd be much simpler than building a dict and updating it based on the signal return values...

Feedback welcome!

---

Also, for our UN colleagues @OmeGak @vasantvohra @SegiNyn, maybe you can have a look as well and see if you'd find this useful? Here's an example on how I'd use this instead of the recently-added draw_item_on_badge signal:

diff --git a/indico/modules/events/registration/badges.py b/indico/modules/events/registration/badges.py
index 918edd21fa..baec526e3a 100644
--- a/indico/modules/events/registration/badges.py
+++ b/indico/modules/events/registration/badges.py
@@ -13,12 +13,11 @@ from reportlab.lib.units import cm
 from reportlab.lib.utils import ImageReader
 from werkzeug.exceptions import BadRequest

-from indico.core import signals
 from indico.modules.designer.pdf import DesignerPDFBase
 from indico.modules.events.registration.settings import DEFAULT_BADGE_SETTINGS
 from indico.util.i18n import _
 from indico.util.placeholders import get_placeholders
-from indico.util.signals import values_from_signal
+from indico.util.signals import make_interceptable


 FONT_SIZE_RE = re.compile(r'(\d+)(pt)?')
@@ -101,15 +100,9 @@ class RegistrantsListToBadgesPDF(DesignerPDFBase):
             else:
                 continue

-            item_data = {'item': item, 'text': text, 'pos_x': pos_x, 'pos_y': pos_y}
-            for update in values_from_signal(
-                signals.event.designer.draw_item_on_badge.send(registration, items=items, height=self.height,
-                                                               width=self.width, data=item_data),
-                as_list=True
-            ):
-                item_data.update(update)
-            self._draw_item(canvas, item_data['item'], tpl_data, item_data['text'], item_data['pos_x'],
-                            item_data['pos_y'])
+            draw_item = make_interceptable(self._draw_item, ctx={'registration': registration, 'items': items,
+                                                                 'height': self.height, 'width': self.width})
+            draw_item(canvas, item, tpl_data, text, pos_x, pos_y)

and the plugin could be doing something like this then:

    def init(self):
        super().init()
        from indico.modules.events.registration.badges import RegistrantsListToBadgesPDF
        from indico.util.signals import interceptable_sender
        self.connect(signals.plugin.interceptable_function, self._intercept_draw_item,
                     sender=interceptable_sender(RegistrantsListToBadgesPDF._draw_item))

    def _intercept_draw_item(self, sender, args, ctx, RETURN_NONE, **kwargs):
        if args.arguments['content'] == 'CERN':
            # this skips the item - if we force a return value, the original function won't get called
            return RETURN_NONE
        elif args.arguments['content']:
            args.arguments['content'] = f"[{args.arguments['content'].strip()}]"
        args.arguments['item']['color'] = '#ff00cc'
        args.arguments['item']['background_color'] = '#000000'