Rename 'default_density' to 'default_linear_damp' in project settings

[Srogozins]

What 'default_density' in project settings actually sets is AREA_PARAM_LINEAR_DAMP. There's already a 'default_angular_damp' setting and this property seems to be mostly referred to as linear damp in the editor and docs, so to avoid confusion I suggest being consistent with the naming.

This change will affect older projects that use 'default_density', but the switch should be very simple.

Test project

[bojidar-bg]

@Srogozins ~ It there a way to make it backward-compatible? Since it would have to wait till 2.1 if it isn't

[Srogozins]

@bojidar-bg I'm not sure. I've tried modifying the change to make it so by applying both options one by one, but that doesn't seem to be work (see diff). I don't really think that it's a critical enough issue to put more effort into keeping compatibility and merging it in can wait till 2.1 instead.

[bojidar-bg]

@Srogozins If you want to try, you can check how GLOBAL_DEF works here: https://github.com/godotengine/godot/blob/master/core/globals.cpp#L1321 .

[akien-mga]

IMO such name changes are fine if it improves the consistency. Godot has this advantage that the user base is not huge enough yet that we need to ensure 100% forward-compatibility for older projects.

On the other hand, we should really start an issue/pad where we document all API name changes between Godot 2.0 and 2.1, so that users can see them easily when upgrading.

[akien-mga]

@Srogozins Sorry for the delay. I think I'll just merge it and start a document to list all API changes that users should be aware of when switching to 2.1.

Could you fix the default_density argument in this demo too? https://github.com/godotengine/godot/blob/23cf6a85bdb904a1833c6015222d9ee69842c1ed/demos/2d/rubegoldberg/engine.cfg

[akien-mga]

Superseded by #5032.