[widgets][assetwidget] Add button to allow closing the current editor

This button allows the user to stop editing the current asset without
having to open a new one.

Fixes #243
4 jobs for 243_add_close_button_to_assetwidget in 12 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #205961
docker
build_linux_36

00:11:57

91.0%
passed #205962
docker
build_linux_37

00:11:38

91.0%
passed #205959
macosx
build_macosx_36

00:10:45

91.0%
passed #205960
macosx
build_macosx_37

00:10:16

91.0%