InstallDialog now returns a InstallQueueItemModel ready
to be downloaded or queued.
Renamed a few model attributes to match legendary's names.
InstallDialog can be run silently for auto-downloads.
The addition of download_only option broke the download queue
due to different positional arguments and tuple indices. Since
using indices is awkward, models to represent the download
and each item in the queue itself were added.
* Fix race condition between the sdl checkboxes.
* Fix default tag not been included in the list of tags.
* Delete InstallDialog on close.
* Specify parent for InstallDialog.
