Resolution and bit depth is shared between all layers and masks

**ArmorPaint version:**
ArmorPaint.org - v0.8 (2021-06-15) - 238ac27

1. Create two or more layers
2. Change the bit depth and/or resolution of any layer
3. It is changed for all layers.

I'm again unsure whether this is by design or not. If it is it is - at least for me - confusing, because the option is in the context menu of layers/masks.

I found this issue while experimenting what is the resolution of a layer group. Currently this is no question but suppose the option would not be shared anymore then it could be the maximum of all layers in the group or something like that.

Start by reproducing the reported behavior in ArmorPaint v0.8 with multiple layers and masks, changing resolution and bit depth from a layer or mask context menu. Read the surrounding layer and layer-group handling to determine the intended scope; done means the behavior is either made explicitly per-layer or the shared behavior is clearly established and documented.

Written by the indexing model from the issue text.