Fixes #1254 so that the current directory is not changed after the used picked files

[philenius]

Important changes:

• Fixes the bug that the current directory sometimes changed after the user picked a file. Now, after the dialog is closed, the current directory is automatically restored to its original value even if the user changed the directory while searching for files (#1254).
• In my previous pull request for v5.2.7, I forgot to commit my Linux implementation 😞 This pull request fixes the behavior of the saveFile() dialog on Ubuntu/Kubuntu. This fix should have been committed in the previous version 5.2.7 but was forgotten. Now, when the user selects an already existing file, then Ubuntu prompts the user to confirm overwriting this file. This change finally makes the dialog behave the same on all desktop platforms (macOS, Linux, and Windows) (#989).

Less important changes:

• I refactored the Windows implementation by replacing parameters by an instance of the class _OpenSaveFileArgs.
  The class _OpenSaveFileArgs has been introduced in f56fac2. However, it wasn't used consistently throughout the whole Windows implementation as each field of _OpenSaveFileArgs was passed individually between functions. Consistenly using an instance of _OpenSaveFileArgs minifies function signatures and reduces the total lines of code by 50 lines.
• Chore: fixed the linter error "unnecessay_import"

  The import of 'dart:typed_data' is unnecessary because all the used elements are also provided by the import of
  package:flutter/foundation.dart'.

• Example app: I re-created our example app and gave it a new Material 3 design. I also added additional input elements to demonstrate the effects of the parameters dialogTitle, initialDirectory, fileName, and lockParentWindow. Sorry for the many new files but they are part of Flutter apps for Windows / Linux and they are not part of the official Flutter .gitignore.

ℹ️ @miguelpruivo, if you you want me to put screenshots/videos from the updated example app into the README, then please let me know. Also, please note that I have not tested the updated example app on iOS because I don't have access to an iPhone.

Windows:

Linux:

linux.mp4

Android:

android.mp4

[miguelpruivo]

@philenius thank you for your effort. If you think it could add some value having the screenshots there, feel free to add so (in reasonable sizes).

[miguelpruivo]

@philenius I have merged to find that some of the readme images are broken now. I’m not sure if you were done with this PR but if not, feel free to add the missing changes and/or I’ll revert it back to the previous version ASAP.

Sorry for any inconvenience.

[miguelslemos]

#1260

[philenius]

@philenius I have merged to find that some of the readme images are broken now. I’m not sure if you were done with this PR but if not, feel free to add the missing changes and/or I’ll revert it back to the previous version ASAP.

Sorry for any inconvenience.

Sorry, my bad, I wasn't done with my PR. I should have marked the PR as a draft.
I created a new PR to fix the broken GIFs and another big. See #1261