[widgets][toolchaineditor] Improve empty loop block dialog

If any of the list shall be empty, set an disabled item stating that
nothing valid is found. This is better than the currently empty dialog.

Part of #239
4 jobs for 259_improve_toolchain_editor in 7 minutes and 12 seconds (queued for 2 seconds)
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passed #199471
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build_linux_36

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passed #199472
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build_linux_37

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build_macosx_37

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