Allow User-Defined Namespace for LayerSurface Creation

[namana-mecha]

What problem does this solve or what need does it fill?

Currently, the namespace for the layer surface created by layer_shell.create_layer_surface is hardcoded to "simple_layer". This prevents users from customizing the namespace to suit their application's needs, potentially leading to naming conflicts or limiting organizational flexibility.

We need to provide an option to allow users to specify the namespace for the layer surface during creation. This would enable greater control over layer surface identification and management.

What solution would you like?

Add an optional namespace field to the LayerShellSettings struct:

pub struct LayerShellSettings {
    // ... other fields ...
    pub namespace: String, // Added namespace field
}

Then, modify layer_shell.create_layer_surface to read this namespace from LayerShellSettings.