wayland/lock: check for protocol availability before use

Fixes #66

src/wayland/session_lock/session_lock.cpp

@@ -22,6 +22,8 @@ QSWaylandSessionLockManager* manager() {
 }
 } // namespace
 
+bool SessionLockManager::lockAvailable() { return manager()->isActive(); }
+
 bool SessionLockManager::lock() {
 	if (this->isLocked() || SessionLockManager::sessionLocked()) return false;
 	this->mLock = manager()->acquireLock();

src/wayland/session_lock/session_lock.hpp

@@ -15,6 +15,8 @@ class SessionLockManager: public QObject {
 public:
 	explicit SessionLockManager(QObject* parent = nullptr): QObject(parent) {}
 
+	[[nodiscard]] static bool lockAvailable();
+
 	// Returns true if a lock was acquired.
 	// If true is returned the caller must watch the global screen list and create/destroy
 	// windows with an attached LockWindowExtension to match it.