[image_picker] Add desktop support - implementations (#4172)

Platform implementation portion of
#3882

Updates the Windows implementation to use the new base class for camera delegation, and creates new macOS and Linux implementations that are near-duplicates.

These are separate packages, rather than a single shared package, because it's likely that they will diverge over time (e.g., the TODO for macOS to use a system image picker control on newer versions of macOS), and the amount of code that could be shared is minimal anyway.

Part of flutter/flutter#102115 Part of flutter/flutter#102320 Part of flutter/flutter#85100

Author: stuartmorgan-g

packages/image_picker/image_picker_linux/AUTHORS

@@ -0,0 +1,7 @@
+# Below is a list of people and organizations that have contributed
+# to the Flutter project. Names should be added to the list like so:
+#
+#   Name/Organization <email address>
+
+Google Inc.
+Alexandre Zollinger Chohfi <[email protected]>

packages/image_picker/image_picker_linux/CHANGELOG.md

@@ -0,0 +1,3 @@
+## 0.2.0
+
+* Implements initial Linux support.

packages/image_picker/image_picker_linux/LICENSE

@@ -0,0 +1,25 @@
+Copyright 2013 The Flutter Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without modification,
+are permitted provided that the following conditions are met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+    * Neither the name of Google Inc. nor the names of its
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
+ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
+ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

packages/image_picker/image_picker_linux/README.md

@@ -0,0 +1,27 @@
+# image\_picker\_linux
+
+A Linux implementation of [`image_picker`][1].
+
+## Limitations
+
+`ImageSource.camera` is not supported unless a `cameraDelegate` is set.
+
+### pickImage()
+The arguments `maxWidth`, `maxHeight`, and `imageQuality` are not currently supported.
+
+### pickVideo()
+The argument `maxDuration` is not currently supported.
+
+## Usage
+
+### Import the package
+
+This package is [endorsed][2], which means you can simply use `file_selector`
+normally. This package will be automatically included in your app when you do,
+so you do not need to add it to your `pubspec.yaml`.
+
+However, if you `import` this package to use any of its APIs directly, you
+should add it to your `pubspec.yaml` as usual.
+
+[1]: https://pub.dev/packages/image_picker
+[2]: https://flutter.dev/docs/development/packages-and-plugins/developing-packages#endorsed-federated-plugin

packages/image_picker/image_picker_linux/example/README.md

@@ -0,0 +1,9 @@
+# Platform Implementation Test App
+
+This is a test app for manual testing and automated integration testing
+of this platform implementation. It is not intended to demonstrate actual use of
+this package, since the intent is that plugin clients use the app-facing
+package.
+
+Unless you are making changes to this implementation package, this example is
+very unlikely to be relevant.