Added tutorial for context menu(desktop)

tutorials/ContextMenu/README.md

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+# Context Menu in Compose for Desktop
+
+## What is covered
+
+In this tutorial we will cover all aspects of work with Context Menu
+using the Compose UI framework.
+
+## Default context menu 
+There is out-of-the box context menu support for TextField and Selectable text. 
+
+To enable standard context menu for a TextField you just need to put it inside DesktopMaterialTheme:
+
+```kotlin
+@OptIn(ExperimentalComposeUiApi::class, androidx.compose.foundation.ExperimentalFoundationApi::class)
+fun main() = singleWindowApplication(title = "Context menu") {
+   DesktopMaterialTheme { //it is mandatory for Context Menu
+       val text = remember {mutableStateOf("Hello!")}
+       TextField(
+           value = text.value,
+           onValueChange = { text.value = it },
+           label = { Text(text = "Input") }
+       )
+   }
+} 
+```
+
+Standard context menu for TextField contains the following items based on text selection: Copy, Cut, Paste, Select All.
+
+Enabling standard context menu for a Text component is similar - you just need to make it selectable: 
+
+```kotlin
+@OptIn(ExperimentalComposeUiApi::class, androidx.compose.foundation.ExperimentalFoundationApi::class)
+fun main() = singleWindowApplication(title = "Context menu") {
+   DesktopMaterialTheme { //it is mandatory for Context Menu
+        SelectionContainer {
+            Text("Hello World!")
+        }
+   }
+} 
+```
+Context menu for text contains just Copy action.
+
+## User-defined context menu
+To enable additional context menu items for TextField and Text components, ContextMenuDataProvider and ContextMenuItem elements are used:
+
+```kotlin
+@OptIn(ExperimentalComposeUiApi::class, androidx.compose.foundation.ExperimentalFoundationApi::class)
+fun main() = singleWindowApplication(title = "Context menu") {
+   DesktopMaterialTheme { //it is mandatory for Context Menu
+       val text = remember {mutableStateOf("Hello!")}
+        Column {
+            ContextMenuDataProvider(
+                items = {
+                    listOf(
+                        ContextMenuItem("User-defined Action") {/*do something here*/},
+                        ContextMenuItem("Another user-defined action") {/*do something else*/}
+                    )
+                }
+            ) {
+                TextField(
+                    value = text.value,
+                    onValueChange = { text.value = it },
+                    label = { Text(text = "Input") }
+                )
+
+                Spacer(Modifier.height(16.dp))
+
+                SelectionContainer {
+                    Text("Hello World!")
+                }
+            }
+        }
+   }
+} 
+```
+In this example Text/TextField context menus will be extended with two additional items. 
+
+## Context menu for arbitary area
+There is possibility to create a context menu for arbitary application window area. This is implemented using ContextMenuArea API that is 
+similar to ContextMenuDataProvider. 
+```kotlin
+@OptIn(ExperimentalComposeUiApi::class, androidx.compose.foundation.ExperimentalFoundationApi::class)
+fun main() = singleWindowApplication(title = "Context menu") {
+   DesktopMaterialTheme { //it is mandatory for Context Menu
+        ContextMenuArea(items = {
+            listOf(
+                ContextMenuItem("User-defined Action") {/*do something here*/},
+                ContextMenuItem("Another user-defined action") {/*do something else*/}
+            )
+        }) {
+            Box(modifier = Modifier.background(Color.Blue).height(100.dp).width(100.dp)) {
+            }
+        }
+   }
+} 
+```
+Right click on the Blue Square will show context menu with two items