Right-click and save model

[chapulina]

As described on the WIP GUI interactions design document, Ignition's model editing mode will be much more integrated into regular simulation than it was on Gazebo-classic. While classic had an entire separate representation of a model in memory for simulation and for editing, on Ignition we plan on leveraging the ECM for both.

In this arrangement, users will never be in a mode where they're editing a single model. So we can't have a workflow like "File menu -> Save model -> Exit editor", or Ctrl + S to save a model. Instead, we'll need to give the user a way to save a specific model at any time. My simple suggestion is to:

• right-click a model
• choose "Save As" - a dialog opens
• choose location and other settings

We should add some other more elaborate hotkeys and buttons in the future to allow the user to quickly iterate while editing a model: edit - save - edit - save...

It would be better to tackle this issue after #137, so that we have a mechanism in place to save meaningful information for the model.