restore cross-build versions

- build with 2.11, 2.12, 2.13, 3.0.0-M1
- update travis
- update readme
- restore MainFrame constructor argument
- remove unused import

Author: Sciss

.travis.yml

@@ -8,6 +8,7 @@ scala:
   - 2.11.12
   - 2.12.11
   - 2.13.3
+  - 3.0.0-M1
 
 env:
   - ADOPTOPENJDK=8

README.md

@@ -5,14 +5,15 @@
 [<img src="https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-swing_2.12.svg?label=latest%20release%20for%202.12"/>](http://search.maven.org/#search%7Cga%7C1%7Cg%3Aorg.scala-lang.modules%20a%3Ascala-swing_2.12)
 [<img src="https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-swing_2.13.svg?label=latest%20release%20for%202.13"/>](http://search.maven.org/#search%7Cga%7C1%7Cg%3Aorg.scala-lang.modules%20a%3Ascala-swing_2.13)
 
-This is now community maintained by [@Sciss](https://github.com/Sciss) and [@benhutchison](https://github.com/benhutchison). If you are interested in helping then contact them or [@adriaanm](https://github.com/adriaanm).
+This is now community maintained by [@Sciss](https://github.com/Sciss) and [@benhutchison](https://github.com/benhutchison). 
+If you are interested in helping then contact them or [@adriaanm](https://github.com/adriaanm).
 
 ## Adding an sbt dependency
 
 To use scala-swing from sbt, add this to your `build.sbt`:
 
 ```
-libraryDependencies += "org.scala-lang.modules" %% "scala-swing" % "2.1.1"
+libraryDependencies += "org.scala-lang.modules" %% "scala-swing" % "2.2.0"
 ```
 
 ## About scala-swing
@@ -113,6 +114,7 @@ new Frame {
   - When using Scala 2.11, you can use the Scala swing 2.0.x releases on JDK 6 or newer.
   - Scala 2.12 requires you to use JDK 8 (that has nothing to do with scala-swing).
 - The `2.1.x` series adds support for Scala 2.13, while dropping Scala 2.10.
+- The `2.2.x` series adds support for Scala 3.0.
 
 The reason to have different major versions is to allow for binary incompatible changes. Also, some java-swing classes were 
 generified in JDK 7 (see [SI-3634](https://issues.scala-lang.org/browse/SI-3634)) and require the scala-swing sources to be adjusted.

build.sbt

@@ -1,7 +1,7 @@
 lazy val scalaTestVersion = "3.2.3"
 
 lazy val commonSettings = Seq(
-  crossScalaVersions += "3.0.0-M1",
+  crossScalaVersions := Seq("3.0.0-M1", "2.13.3", "2.12.12", "2.11.12"),
   scalaVersion       := "2.13.3",
   scalacOptions     ++= Seq("-deprecation", "-feature"),
 )

src/main/scala/scala/swing/Action.scala

@@ -12,8 +12,6 @@
 
 package scala.swing
 
-import java.awt.event.ActionListener
-
 import javax.swing.{Icon, KeyStroke}
 
 object Action {

src/main/scala/scala/swing/MainFrame.scala

@@ -16,6 +16,6 @@ package scala.swing
  * A frame that can be used for main application windows. It shuts down the
  * framework and quits the application when closed.
  */
-class MainFrame(/*gc: java.awt.GraphicsConfiguration = null*/) extends Frame /*(gc)*/ {
+class MainFrame(gc: java.awt.GraphicsConfiguration = null) extends Frame(gc) {
   override def closeOperation(): Unit = sys.exit(0)
 }

src/main/scala/scala/swing/Publisher.scala

@@ -79,7 +79,7 @@ private[swing] trait LazyPublisher extends Publisher {
 
 import scala.ref._
 
-private[swing] trait SingleRefCollection[/*+*/ A <: AnyRef] extends Iterable[A] { self =>
+private[swing] trait SingleRefCollection[A <: AnyRef] extends Iterable[A] { self =>
 
   trait Ref[+B <: AnyRef] extends Reference[B] {
     override def hashCode(): Int = get match {