Implement EditorApplication type

[AlmasB]

In addition to games (which extend GameApplication), FXGL can be used to develop editors. Example: dialogue editor. Editors typically share some functionality:

• An infinite grid that can be traversed by clicking and dragging the mouse
• Zoom in and out on a specific point
• Z-axis rotations
• Select 1 or more items in the editor
• Change properties of selected items
• Toolbar
• Preferences

Therefore, it could be useful to implement these into an EditorApplication supertype. This type is likely to live in a new module that depends on fxgl, similar to the dialogue editor module. Some of the dialogue editor code will be useful here. After this new module is ready, the dialogue editor can extend from it rather than from GameApplication.

• Also need to consider how to generalize MouseGestures from dialogue editor to take into account rotate/scale

Any thoughts / ideas / suggestions on this are welcome.