Export GLTF instead of FBX, and use GLTF settings.

[Nightriff]

GLTF is a more open standard than FBX, and is therefore the preferred format for better interoperability between programs. Now that the OpenCloud endpoint supports gltf, we can transition this add-on to use it as well.

Blender supports a different set of settings for GLTF than FBX. This PR replaces the FBX settings in AddonPreferences with GLTF settings. Similarly to how FBX settings were ripped from Blender's core exporter add-on, the same is done for GLTF settings to support the same export settings as the regular export flow. A subset of settings have been removed in favor of add-on defaults, such as restrictions on which objects get exported and removal of deprecated settings.

Additionally, a few defaults have are changed (e.g. blender defaults to not Apply Modifiers during export; this add-on defaults to True, since it seems more useful to do so).

Instead of cluttering index.py, any settings that were pulled from blender's core add-on have been placed in their own separate private dependency file.

The overall functionality of the add-on remains largely the same, except for a different set of export settings to work with.

Granted, it's difficult to test every setting available here to ensure it works, because there are so many. I have smoke tested a few of them and all the ones I tried worked just fine. They're simply passed directly into the export operator, so it should just work.

The new setting also includes a +Y Axis Up setting, which implements #13