[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 Name Job ID Coverage
  Build
passed build_linux_36 #205961
docker

00:11:57

91.0%
passed build_linux_37 #205962
docker

00:11:38

91.0%
passed build_macosx_36 #205959
macosx

00:10:45

91.0%
passed build_macosx_37 #205960
macosx

00:10:16

91.0%