Allow custom command lines to reveal files in file browser

[Oliver-Loeffler]

As of now, Scene Builder uses reasonable hard coded platform defaults to reveal files.
On Windows this is limited to explorer /select,<path>, on macOS it uses open -R <path> and on Linux it relies on xdg-open.

Not every Linux system provides xdg-utils and not every user likes Windows Explorer.
So instead of providing multiple fallback options in Scene Builder, a user could manually configure the desired tool of choice for file browsing in Scene Builder preferences. Furthermore, in case of errors, users can change the command line to make Scene Builder work as desired.

Expected Behavior

• Scene Builder allows to manually override the default command line for revealing files in file system
• Scene Builder allows to manually override the default command line for opening files in file system
• Scene Builder allows to manually override the default command line for opening URLs

Current Behavior

• Scene Builder makes use of reasonable defaults.
• Defaults are hard coded.
• User cannot change the command lines.

Context

On Linux Scene Builder requires xdg-utils installed. However, the installation package has no dependency to xdg-utils.
Now, if xdg-utils is not installed, the open URL function and the reveal file functions wont work. Also, opening files from within Scene Builder wont work. The bad thing is, the user has no chance to repair this. On Windows for example, one might use FreeCommander or TotalCommander, same for macOS.

Having a setting for this in Preferences dialog would be helpful for users to adjust these settings for their needs or to repair if the default values wont work. The preferences dialog should be defined in a way, so that the user can revert to defaults any time.

There are command lines for:

• FXML file opening (for editing)
• Reveal a files location in file system browser
• Opening a remote URL

Related issues

#674