resolution: add signal for active resolution changes

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

Reference #8.

README.md

@@ -66,6 +66,8 @@ The **org.freedesktop.ratbag1.Resolution** interface provides:
 - Methods:
   - SetResolution(uint, uint) -> x/y resolution to assign
   - SetReportRate(uint) -> uint for the report rate to assign
+- Signals:
+  - ActiveResolutionChanged -> active resolution changed, carries index of the currently active resolution
 
 The **org.freedesktop.ratbag1.Button** interface provides:
 - Properties:

src/ratbagd-resolution.c

@@ -86,6 +86,14 @@ static int ratbagd_resolution_set_resolution(sd_bus_message *m,
 		resolution->xres = xres;
 		resolution->yres = yres;
 	}
+
+	(void) sd_bus_emit_signal(sd_bus_message_get_bus(m),
+				  "/org/freedesktop/ratbag1",
+				  "/org.freedesktop.ratbag1.Resolution",
+				  "ActiveResolutionChanged",
+				  "u",
+				  resolution->index);
+
 	return sd_bus_reply_method_return(m, "u", r);
 }
 
@@ -133,6 +141,7 @@ const sd_bus_vtable ratbagd_resolution_vtable[] = {
 	SD_BUS_PROPERTY("ReportRate", "u", NULL, offsetof(struct ratbagd_resolution, rate), SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
 	SD_BUS_METHOD("SetReportRate", "u", "u", ratbagd_resolution_set_report_rate, SD_BUS_VTABLE_UNPRIVILEGED),
 	SD_BUS_METHOD("SetResolution", "uu", "u", ratbagd_resolution_set_resolution, SD_BUS_VTABLE_UNPRIVILEGED),
+	SD_BUS_SIGNAL("ActiveResolutionChanged", "u", 0),
 	SD_BUS_VTABLE_END,
 };