Fix TileMap layer reverts and defaults

[KoBeWi]

I noticed that in beta2 my TileMaps suddenly started writing navigation_enabled = true property in the scene. Which is pointless, because it's a default value™.
Then I noticed that this actually applies to all TileMap layer properties. They write their default value, but only if the layer is higher than 0...

I fixed this inconsistency and all layers will now skip defaults. Before:

[node name="TileMap" type="TileMap" parent="."]
tile_set = SubResource("TileSet_mr1hv")
format = 2
layer_0/name = "Test"
layer_0/z_index = 5
layer_0/tile_data = PackedInt32Array(262148, 0, 0)
layer_1/name = "Fest"
layer_1/enabled = true
layer_1/modulate = Color(1, 1, 1, 1)
layer_1/y_sort_enabled = false
layer_1/y_sort_origin = 0
layer_1/z_index = 1
layer_1/navigation_enabled = true
layer_1/tile_data = PackedInt32Array()
script = SubResource("GDScript_j14su")

After:

[node name="TileMap" type="TileMap" parent="."]
tile_set = SubResource("TileSet_mr1hv")
format = 2
layer_0/name = "Test"
layer_0/z_index = 5
layer_0/tile_data = PackedInt32Array(262148, 0, 0)
layer_1/name = "Fest"
layer_1/tile_data = PackedInt32Array()
script = SubResource("GDScript_j14su")

(tile_data is always saved, though it can be easily changed too. I just wanted to avoid comparing big arrays)

As a bonus, you can also revert the layer properties now (previously it only worked for the first layer):

godot.windows.editor.dev.x86_64_18ULSnpU73.mp4

[groud]

I am not sure about the implementation. It seems a bit too complex for what it does.
I remember reduz mentioning a _validate_property callback that can be used for that, maybe that would be the way to go ?

Otherwise I guess it could be done that way but I kind of dislike having to add 80 lines of code just to define some properties defaults.

[KoBeWi]

_validate_property() is not called for properties added with _get_property_list().
And even if it was, you can't use it to define defaults and reverts. The code would be mostly the same, with an extra method that duplicates component check.

Dynamic properties not being revertable is actually a bigger issue and I'm thinking that it can be handled with a helper class (it would at least allow writing much less code). But it's totally out of scope of this PR.

[akien-mga]

@groud I think you discussed this further on chat, is this good to approve now?

[akien-mga]

Thanks!