Provide preset-like factory functions for creating targets in new MPP

* Pull the `targets` and `presets` properties of the `kotlin` extension
  up to an interface in `kotlin-gradle-plugin-api`

* Generate the code: we need type-safe statically-typed functions for
  different target type, which seems to be only possible to achieve by
  providing a function per preset.

* Place these functions in the top-level `kotlin` Gradle extension,
  rather than extension functions for `kotlin.targets`:

  - `kotlin.jvm { ... }` will work, while `kotlin.targets.jvm { ... }`
    won't, the extension function needs to be somehow brought into scope

  - reducing the nesting level by one seems to be a good move here

Issue #KT-26389 Fixed

libraries/tools/kotlin-gradle-plugin-dsl-codegen/build.gradle.kts

@@ -0,0 +1,28 @@
+import org.gradle.jvm.tasks.Jar
+
+plugins {
+    kotlin("jvm")
+}
+
+dependencies {
+    compile(gradleApi())
+    compile(project(":kotlin-gradle-plugin-api"))
+    compile(project(":kotlin-native:kotlin-native-utils"))
+}
+
+val generateMppTargetContainerWithPresets by generator(
+    "org.jetbrains.kotlin.generators.gradle.dsl.MppPresetFunctionsCodegenKt",
+    sourceSets["main"]
+)
+
+generateMppTargetContainerWithPresets.run {
+    systemProperty(
+        "org.jetbrains.kotlin.generators.gradle.dsl.outputSourceRoot",
+        project(":kotlin-gradle-plugin").projectDir.resolve("src/main/kotlin").absolutePath
+    )
+}
+
+// Workaround: 'java -jar' refuses to read the original dotted filename on Windows, 'Unable to access jarFile org.jetbrains.kotlin....jar'
+tasks.named<Jar>(generateMppTargetContainerWithPresets.name + "WriteClassPath").configure {
+    archiveName = generateMppTargetContainerWithPresets.name
+}