Added a new reusable FileSelector control allowing tools to get one or many files as input.

[veler]

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• Bugfix
• Feature
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What is the current behavior?

More and more tools are getting the ability to accept files as an input, in particular tools in the Graphic category.
Unfortunately, each tools were copying the UI and implementation for letting the user selecting a file, leading to a bunch of duplicated code.

Issue Number: #244, #151

What is the new behavior?

Created a control FileSelector which basically unify the UI and implementation for such a need.
The control has several parameters:

• AllowedFileExtensions - Defines the exhaustive list of file type that the user can select through the control. Using "*" or a null value corresponds to allowing any kind of file.
• AllowMultipleFileSelection - Tells whether the user will be able to select a single or multiple files. Changing this value changes the way the Drag & Drop instructions are displayed. Setting this value to true also display a Browse folders option, which allows the user to select a folder. The control will then pick up the files in this folder (but not sub-folders) that match AllowedFileExtensions.
• AllowPasteImage - The "Paste" button allows the user to paste one or several files that he has in the clipboard. Enabling this option will additionally allow the user to paste an image from the clipboard such as a screenshot.
• FilesSelectedCommand - A MVVM command that is invoked once the user selected a file, so the tool can reacts to the user interaction.

The control filters automatically the selected / dropped / pasted files based on the AllowedFileExtensions and shows a message when the files don't respect the filter. However, it doesn't show this message when selecting a folder.

Here is how to use the control:

<controls:FileSelector
            AllowedFileExtensions="png;jpg;jpeg;bmp"
            AllowMultipleFileSelection="False"
            AllowPasteImage="True"
            FilesSelectedCommand="{x:Bind ViewModel.FilesSelectedCommand}"/>
<!-- or -->
<controls:FileSelector
            AllowedFileExtensions="*"
            FilesSelectedCommand="{x:Bind ViewModel.FilesSelectedCommand}"/>

        FilesSelectedCommand = new RelayCommand<StorageFile[]>(ExecuteFilesSelectedCommand);

        [...]

        public IRelayCommand<StorageFile[]> FilesSelectedCommand { get; } = new RelayCommand<StorageFile[]>(ExecuteFilesSelectedCommand);

        private void ExecuteFilesSelectedCommand(StorageFile[]? files)
        {
            if (files is not null)
            {
                foreach (StorageFile file in files)
                {
                    QueueNewConversion(file);
                }
            }
        }

Other information

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• Followed the code style guideline as described in CONTRIBUTING.md
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