Refactor: apply best practices and coding guidelines globally (OpenEMS#2209)

The PR contains strictly **NO LOGIC CHANGES**, but applies best practices and coding guidelines (https://openems.github.io/openems.io/openems/latest/contribute/coding-guidelines.html) globally in OpenEMS Edge/Backend, e.g.

- Use separate Interface and Implementation files for OpenEMS Edge Components
- Move ChannelIds to Interfaces ("Natures" - https://openems.github.io/openems.io/openems/latest/coreconcepts.html#_nature)
- Rename Component files equal to Factory-ID (makes Ctrl+R in Eclipse IDE much more convenient to use)
- Reduce variable scope; use `private` wherever possible (e.g. for the `activate()`-method). This is enforced by adding and applying Checkstyle `VisibilityModifierCheck` (see https://checkstyle.sourceforge.io/apidocs/com/puppycrawl/tools/checkstyle/checks/design/VisibilityModifierCheck.html)
- Sort variables in code (from `public static final` to `private`; @reference to constructor to @activate, @Modifed, @deactivate)
- Use short JavaDoc for single-line comments
- Use `@EventTopics`
- Always implement @deactivate
- Remove `package-info.java` if not necessary (i.e. reduce package scope)
- Add (at least simple) JUnit tests
- Update BND Bundle templates

Author: sfeilmeier

cnf/checkstyle.xml

@@ -188,7 +188,9 @@
       <message key="name.invalidPattern" value="Method name ''{0}'' must match pattern ''{1}''."/>
     </module>
     <module name="SingleLineJavadoc">
+      <property name="severity" value="ignore"/>
       <property name="ignoreInlineTags" value="false"/>
+      <metadata name="net.sf.eclipsecs.core.lastEnabledSeverity" value="inherit"/>
     </module>
     <module name="EmptyCatchBlock">
       <property name="exceptionVariableName" value="expected"/>
@@ -206,6 +208,11 @@
       <property name="ignoreMethodNamesRegex" value="doc|set.+|build"/>
       <property name="tokens" value="METHOD_DEF"/>
     </module>
+    <module name="VisibilityModifier">
+      <property name="protectedAllowed" value="true"/>
+      <property name="allowPublicFinalFields" value="true"/>
+      <property name="allowPublicImmutableFields" value="true"/>
+    </module>
   </module>
   <module name="LineLength">
     <property name="severity" value="ignore"/>

io.openems.backend.alerting/src/io/openems/backend/alerting/Alerting.java

@@ -41,8 +41,9 @@ public class Alerting extends AbstractOpenemsBackendComponent implements EventHa
 	private static final byte THREAD_POOL_SIZE = 2;
 	private static final byte THREAD_QUEUE_WARNING_THRESHOLD = 50;
 
-	private final Logger log = LoggerFactory.getLogger(Alerting.class);
+	protected final Scheduler scheduler;
 
+	private final Logger log = LoggerFactory.getLogger(Alerting.class);
 	private final ThreadPoolExecutor executor;
 
 	@Reference
@@ -51,7 +52,6 @@ public class Alerting extends AbstractOpenemsBackendComponent implements EventHa
 	@Reference
 	protected Mailer mailer;
 
-	protected final Scheduler scheduler;
 	protected Handler<?>[] handlers = {};
 
 	protected Alerting(Scheduler scheduler) {