ProjectExplorer: Let toolchains be (de)registered in bulk

When some code adds or removes a bunch of toolchains at once, that
should be made explicit and also be reflected by the respective signal
emissions.
Fix some leaks and typos along the way.

Change-Id: I4a1f12a2378316c5faf70e85a88adc467f076caf
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/toolchain.cpp

@@ -787,14 +787,16 @@ void AsyncToolchainDetector::run()
                      [watcher,
                       alreadyRegistered = m_alreadyRegistered]() {
                          Toolchains existingTcs = ToolchainManager::toolchains();
+                         Toolchains toRegister;
                          for (Toolchain *tc : watcher->result()) {
                              if (tc->isValid() && !alreadyRegistered(tc, existingTcs)) {
-                                 ToolchainManager::registerToolchain(tc);
+                                 toRegister << tc;
                                  existingTcs << tc;
                              } else {
                                  delete tc;
                              }
                          }
+                         ToolchainManager::registerToolchains(toRegister);
                          watcher->deleteLater();
                      });
     watcher->setFuture(Utils::asyncRun(m_func, m_detector));