make boxes (files/folders) removable via contextMenu

[MichiSpebach]

add remove option to src/core/contextMenu.ts for FileBoxes (and FolderBoxes)

Here little bit more has to be done:

• add abstract remove(path: string): Promise<void> to interface src/core/fileSystemAdapter.ts
• implement it for src/electronApp/NodeJsFileSystemAdapter.ts
• implement it for src/browserApp/BrowserFileSystemAdapter.ts
• add and implement remove option in src/core/contextMenu.ts
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