Special support for editor apps

[mastersign]

Apps which can edit text files, or open directories as an IDE, should be known to the Bench system.
The Bench system can then provide a user interface for opening the default editor with an empty file, a specific file or a whole directory/project.

The following cases should be distinguished, because the user might prefer different apps for them:

• Opening a directory in an editor / IDE
• Opening one or multiple text files in an editor

The following use cases are considered:

• Bench CLI command to open a directory in the default editor
• Bench CLI command to open one ore multiple text files in the default editor
• open-dir.cmd script on the PATH to open a directory in the default editor
• open-text.cmd script on the PATH to open one ore multiple text files in the default editor
• Optionally register a context menu entry Bench Editor for the Windows Explorer in HKCU - for directories and files, respectively
• Button in the Bench Dashboard to open the default directory editor
  (with drag-and-drop support)
• Button in the Bench Dashboard to open the default text editor
  (with drag-and-drop support)

For project support the following use cases are considered:

• Opening a project directory in the default editor
• Opening a file from the project tree in the default editor

[mastersign]

• Default editor for the whole Bench environment
• Default editor for an individual project

[mastersign]

Added config properties TextEditorApp, MarkdownEditorApp in 59c5e3d.

[mastersign]

Removed config property MarkdownEditorApp in 48b4e98.