Physics2D: Rename default_density to default_linear_damp

[akien-mga]

Also adds a proper deprecating warning and tries to do some porting automatically.
Supersedes #4109.

@reduz @neikeq What do you think of the way I added the deprecation warning? Should I make this a method of Globals, e.g. set_and_deprecate("physics_2d/default_linear_damp","physics_2d/default_density")? Related to #4397.

[akien-mga]

I've tested it on a project with default settings (no persisting physics_2d/default_density) and with a custom physics_2d/default_density, it works as expected.

The only issue is for projects which would be defining physics_2d/default_density directly via GDScript, but since we'll document this change in the release notes, it should be OK.

[neikeq]

Your method seems fine in this case, but what about deprecating methods that have no such property in project settings?

[akien-mga]

Your method seems fine in this case, but what about deprecating methods that have no such property in project settings?

For this we still need to go forward with what was discussed on #4397 and implement a proper method. In #4571 I duplicated all deprecated methods to add a warning when they're called, but it's tedious and a global method or macro would be better for that.