[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 Name Job ID Coverage
  Build
passed build_linux_36 #205609
docker

00:11:29

90.0%
passed build_linux_37 #205610
docker

00:11:15

90.0%
passed build_macosx_36 #205607
macosx

00:10:28

90.0%
passed build_macosx_37 #205608
macosx

00:10:05

90.0%