powermenu: add soft reboot option for systemd systems

fixes #2986
port 1.5

(cherry picked from commit e089948225)

quickshell/Services/SessionService.qml

@@ -17,6 +17,7 @@ Singleton {
     property bool loginctlCommandAvailable: false
     property bool systemctlCommandAvailable: false
     property bool hibernateSupported: false
+    readonly property bool softRebootSupported: systemctlCommandAvailable
     property bool inhibitorAvailable: true
     property bool idleInhibited: false
     property string inhibitReason: "Keep system awake"
@@ -452,6 +453,10 @@ Singleton {
         }
     }
 
+    function softReboot() {
+        Quickshell.execDetached(["systemctl", "soft-reboot"]);
+    }
+
     function poweroff() {
         if (SettingsData.customPowerActionPowerOff.length === 0) {
             Quickshell.execDetached(powerManagerCommand("poweroff"));