Various breaking changes where done, leading to errors in the
application template. This fixes all I could find.
Addresses:
- `bgl` removal
- Removal of `2D_` prefixes in shader names
- Removal of context override argument for operators in favor of
`context.temp_override()`
- Renamed/repurposed `FileSelectParams.display_size`
See https://wiki.blender.org/wiki/Reference/Release_Notes/4.0/Python_API.
