Crumble improvements

[Strilanc]

• Make 'K+#' autocomplete work with MXX and MPP instructions
• Make 'Shift+K+#' add an unused dissipative operation to a flow
• Add a 'remove detectors' button
• Do something more reasonable when attempting to convert a bad flow into a detector and vice versa
• Make 'convert to observable' pick the smallest unused obs index