Allow background execution of GDE tools

[westram]

Allow adding "run in background" option to GDE GUI header.

If set

• create tempdir including a FILE for communication between arb and subprocess. Store name of FILE together with current date in arb .
• disconnect subprocess from arb_launcher (call like ​arb daemons). After subprocess has terminated, store success/failure information in FILE.
• ARB_NT should watch such FILEs for updates (e.g. using inotify) - even after restarting . On change, ARB_NT shall report error or import results.

In lib/gde/*.menu split itemmethod into three phases:

1. export/startup
2. main
3. import/shutdown

Phase 1. and 3. require running ARB, phase 2. should work w/o ARB. All phases get started by ARB.

Differences between aligners and treeing-tools:

• treeing-tools already reimport the generated asynchronously
  • will work fine with background calculation
  • name issue (see #461) has to be solved
• when calling an aligner, GDE currently waits for it to finish. It doesn't make sense to work with an alignment as long an aligner is working on it in the background, so background execution should be restricted to de-novo alignment (see also #343)

[westram]

Milestone renamed