[widgets][dialogs] Make new dialog comboboxes searchable

This patch makes the combo boxes editable so the user
can start typing what he is looking for rather than
having to scroll a possibly long list of assets.

Fixes #227
4 jobs for 227_make_new_dialog_comboboxes_searchable in 11 minutes and 30 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #205609
docker
build_linux_36

00:11:29

90.0%
passed #205610
docker
build_linux_37

00:11:15

90.0%
passed #205607
macosx
build_macosx_36

00:10:28

90.0%
passed #205608
macosx
build_macosx_37

00:10:05

90.0%