Move CueGUI constants to a YAML config file.

[bcipriano]

Link the Issue(s) this Pull Request is related to.
#785

Summarize your change.
Prior to this change, most of the values in CueGUI's Constants.py could only be changed by editing the code directly. For some of those values, that was fine, as they are true constants, but that file contains a lot of settings that should be easily tweakable by users -- refresh settings, style settings, render log roots, plugin paths, etc.

This change creates a new cuegui.yaml file where all of that config can live:

• Constants.py is kept to ease the transition, for now at least.
• Config is stored in YAML format.
• Default config is stored in config/cuegui.yaml. This file also provides an example users can work from to create their own config.
• Custom config will be loaded from:
  • ~/.config/opencue/cuegui.yaml on Linux/Mac.
  • %APPDATA%/opencue/cuegui.yaml on Windows.
  • An arbitrary path specified by the CUEGUI_CONFIG_FILE env var.
• Config.py is renamed to Layout.py, as it only really deals with saving/restoring window layout, so it was confusing.
• We introduce a new constant CONFIG_PATH. Currently DEFAULT_INI_PATH is used for all config files, not just .ini files, which is confusing.
• Removed any constants that weren't referenced elsewhere in the code to clean things up. There were about 10 of them.
• Merged the existing cue_resources.yaml into the new YAML file.

[DiegoTavares]

LGTM