Make GUI part of Engine and move Disk reader/writer to Engine

This is a first step in making a headless run of Loopp possible. The GUI gets an
independend member of the Engine. This way we can decide on runtime wheather to create
or not.
5 jobs for 48-add-core-object in 1 minute and 19 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed #2638
docker
build-publish

00:00:31

passed #2637
docker
build-test

00:00:22

 
  Test
failed #2639
docker allowed to fail
static-analysis

00:00:10

passed #2640
docker
test

00:00:22

71.0%
 
  Package
passed #2641
docker
package-AppImage

00:00:24

 
Name Stage Failure
failed
static-analysis Test
      46     12    439      2      74 Looper::process@111-184@src/looper.cxx
62 21 400 2 93 GridLogic::pressed@129-221@src/gridlogic.cxx
65 13 519 2 99 gui_header_callback@100-198@src/gui.cxx
328 79 2878 1 342 Gui::keyboardHandler@552-893@src/gui.cxx
==========================================================================================
Total nloc Avg.NLOC AvgCCN Avg.token Fun Cnt Warning cnt Fun Rt nloc Rt
------------------------------------------------------------------------------------------
19610 14.4 2.9 102.0 616 31 0.05 0.34
ERROR: Job failed: exit code 1