profile: add signal for active profile changes

This implements a signal that is fired whenever the active profile
changes. For now the signal is only fired when the profile changes
through the setter, ratbagd_profile_set_active, as profile changes may
not always be hooked up in the hardware.

Fixes #7.

README.md

@@ -53,6 +53,8 @@ The **org.freedesktop.ratbag1.Profile** interface provides:
 - Methods:
   - SetActive(void) -> set this profile to be the active profile
   - GetResolutionByIndex(uint) -> returns the object path for the given index
+- Signals:
+  - ActiveProfileChanged -> active profile changed, carries index of the currently active profile
 
 The **org.freedesktop.ratbag1.Resolution** interface provides:
 - Properties:

src/ratbagd-profile.c

@@ -245,6 +245,13 @@ static int ratbagd_profile_set_active(sd_bus_message *m,
 	if (r < 0)
 		return r;
 
+	(void) sd_bus_emit_signal(sd_bus_message_get_bus(m),
+				  "/org/freedesktop/ratbag1",
+				  "/org.freedesktop.ratbag1.Profile",
+				  "ActiveProfileChanged",
+				  "u",
+				  profile->index);
+
         return sd_bus_reply_method_return(m, "u",
                                           ratbag_profile_set_active(profile->lib_profile));
 }
@@ -279,6 +286,7 @@ const sd_bus_vtable ratbagd_profile_vtable[] = {
 	SD_BUS_PROPERTY("DefaultResolution", "u", ratbagd_profile_get_default_resolution, 0, 0),
 	SD_BUS_METHOD("SetActive", "", "u", ratbagd_profile_set_active, SD_BUS_VTABLE_UNPRIVILEGED),
 	SD_BUS_METHOD("GetResolutionByIndex", "u", "o", ratbagd_profile_get_resolution_by_index, SD_BUS_VTABLE_UNPRIVILEGED),
+	SD_BUS_SIGNAL("ActiveProfileChanged", "u", 0),
 	SD_BUS_VTABLE_END,
 };