Allow multiple Godot Editor Executables #969

Problem statement

Currently, Godot Tools assumes that users will have only one version of the Godot Editor installed as their development environment. This assumption leads to usability issues for those debugging builds or maintaining multiple versions of Godot for different purposes (for example, one custom-built from plugin vendors for the main project and another official release used for testing and contributing).

Proposed solution

Allow the user to specify an array of paths to the Godot Editor executables; when opening a workspace that contains a project.godot:

1. Read the first value that matched regex('\d+\.\d+') under the application/config/features as the version number.
2. Read the value that matched C# under the application/config/features as the C# feature toggle.
3. Query the list of executables under the provided array, call each executable with the --version argument for the version number, and whether it's a mono version
4. Choose the first executable that is strictly compatible with the project as the Godot Editor selection.
5. If none is applicable, pop up the error dialog.