External drag and drop support for Desktop (#391)

---------

Co-authored-by: Igor Demin <igordmn@users.noreply.github.com>

compose/desktop/desktop/samples/src/jvmMain/kotlin/androidx/compose/desktop/examples/dnd/Main.jvm.kt

@@ -0,0 +1,89 @@
+/*
+ * Copyright 2023 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package androidx.compose.desktop.examples.dnd
+
+import androidx.compose.foundation.Image
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.material.MaterialTheme
+import androidx.compose.material.Text
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.DragData
+import androidx.compose.ui.ExperimentalComposeUiApi
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.graphics.painter.Painter
+import androidx.compose.ui.onExternalDrag
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.window.singleWindowApplication
+
+@OptIn(ExperimentalComposeUiApi::class)
+fun main() = singleWindowApplication(
+    title = "External dnd demo"
+) {
+    MaterialTheme {
+        val size = 200.dp
+
+        var isDragging by remember { mutableStateOf(false) }
+        var text by remember { mutableStateOf<String?>(null) }
+        var painter by remember { mutableStateOf<Painter?>(null) }
+
+        Column(modifier = Modifier.padding(20.dp)) {
+            Box(
+                modifier = Modifier.size(size, size)
+                    .background(
+                        when {
+                            isDragging -> Color.Green
+                            text != null -> Color.White
+                            else -> Color.Red
+                        }
+                    )
+                    .onExternalDrag(
+                        onDragStart = {
+                            isDragging = true
+                        },
+                        onDragExit = {
+                            isDragging = false
+                        },
+                        onDrag = {
+
+                        },
+                        onDrop = { state ->
+                            val dragData = state.dragData
+                            text = dragData.toString()
+                            if (dragData is DragData.Image) {
+                                painter = dragData.readImage()
+                            }
+                            isDragging = false
+                        })
+            ) {
+                Text(text ?: "Try to drag some files or image here", modifier = Modifier.align(Alignment.Center))
+                val currentPainter = painter
+                if (currentPainter != null) {
+                    Image(currentPainter, contentDescription = "Pasted Image")
+                }
+            }
+        }
+    }
+}