[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)
Status Name Job ID Coverage
  Build
passed build_linux_36 #199471
docker

00:06:40

89.0%
passed build_linux_37 #199472
docker

00:06:36

89.0%
passed build_macosx_36 #199469
macosx

00:07:10

89.0%
passed build_macosx_37 #199470
macosx

00:06:58

89.0%