Add sample plugin for Task API

samples/tasks-plugin/.gitignore

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+dist/
+node_modules/
+*.theia

samples/tasks-plugin/README.md

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+# tasks-plugin
+
+Sample task plugin which includes:
+
+- Providing custom tasks. 
+- Resolving tasks before starting these ones.
+- Ability to listening to events for a task execution starting and completion of an executed task.
+- Ability to get currently active task executions.
+- Displaying running tasks in the bottom panel.
+- Ability to terminate an running task.

samples/tasks-plugin/package.json

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+{
+      "name": "tasks-plugin",
+      "publisher": "theia",
+      "keywords": [
+        "theia-plugin"
+      ],
+      "version": "0.0.1",
+      "license": "EPL-2.0",
+
+      "files": [
+        "src"
+      ],
+      "devDependencies": {
+        "@theia/plugin": "next",
+        "@theia/plugin-packager": "latest",
+        "rimraf": "2.6.2",
+        "typescript-formatter": "7.2.2",
+        "typescript": "2.9.2",
+        "ts-loader": "^4.1.0",
+        "clean-webpack-plugin": "^0.1.19",        
+        "webpack": "^4.1.1",
+        "webpack-cli": "^3.1.1"
+      },
+      "scripts": {
+        "prepare": "yarn run clean && yarn run build",
+        "clean": "rimraf lib",
+        "format-code": "tsfmt -r",
+        "watch": "webpack-cli -w --config webpack.config.js",
+        "compile": "webpack-cli --config webpack.config.js",
+
+        "build": "yarn run format-code && yarn run compile && theia:plugin pack"
+      },
+      "engines": {
+        "theiaPlugin": "next"
+      },
+      "theiaPlugin": {
+            "frontend": "dist/tasks-plugin-frontend.js"
+      }
+}

samples/tasks-plugin/resources/task-long-running

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+#!/bin/bash
+
+for i in {1..200}
+do
+   sleep 1
+   echo "processing... $i"
+done
+