Declare new jobs in configuration file

[KernAttila]

Is your feature request related to a problem? Please describe.
To create new jobs, one has to author some code, making it mandatory to fork the codebase and inevitably drifting away from future updates.

Describe the solution you'd like
Declaring jobs and their associated command should be as simple as adding a new entry in a config file.
In order to accomodate any configuration, we need a way to parse the job configuration and build widgets accordingly.
Additionally, the main cuesubmit config file should be declared as an environment variable to ease distribution and have an option to have sub-config files (see example below)
In case we need to use tokens, they should be hidden from the UI.

Some new widgets should be added (int slider, float slider, file browser, folder browser)
And finally, a command feedback widget to display the final command would be very handy.

Example
Here is an example config file :

RENDER_CMDS:
  Sub config job example:
    config_file: "%MAYA_JOB_CONFIG_FILE%"

  Maya example:
    command: "Render"
    options:
      "-cam {camera}": "persp"
      "-s {startFrame}": "#FRAME_START#"
      "-e {endFrame}": "#FRAME_END#"
      "-log {logLevel}": [0, 5, 3]
      "-renderer {engine}":
          - arnold
          - default
          - hw2
          - sw
      "{mayaFile*}":
          - Maya Ascii file (*.ma)
          - Maya Binary file (*.mb)
          - Maya file (*.ma *.mb)

  Shell example:
    command: ""
    options:
      "{Command to run}": "\n"