Implements https://github.com/godotengine/godot-proposals/issues/1835#issuecomment-727186192
* PauseMode is now ProcessMode, containing the following states:
```
PROCESS_MODE_INHERIT, // same as parent node
PROCESS_MODE_NORMAL, // process only if not paused
PROCESS_MODE_PAUSE_ONLY, // process only if paused
PROCESS_MODE_ALWAYS, // process always
PROCESS_MODE_DISABLED, // never process
```
* NOTIFICATION_PAUSED and NOTIFICATION_UNPAUSED are received effectively when the node is paused and unpaused (not any longer when pause mode is set in SceneTree).
* Renamed some nodes that used ProcessMode/process_mode to specify a callback type to ProcessCallback to avoid clashes.