Fix #136359: Add data-block selector to handle missing texture better

Instead of only showing a button to switch to the texture properties -
which don't actually allow creating a texture for linestyle - add a
data-block selector that allows creating a texture for linestyle. It
will also show a button to switch to the texture properties, if
available.

It seems like the previous UI didn't have any good way of assigning a
texture for linestyle, without going into the texture node editor.

Pull Request: https://projects.blender.org/blender/blender/pulls/141408

scripts/startup/bl_ui/properties_freestyle.py

@@ -1247,12 +1247,11 @@ class VIEWLAYER_PT_freestyle_linestyle_texture(ViewLayerFreestyleLineStyle, Pane
         layout.prop(linestyle, "texture_spacing", text="Spacing Along Stroke")
 
         row = layout.row()
-        props = row.operator(
-            "wm.properties_context_change",
-            text="Go to Linestyle Textures Properties",
-            icon='TEXTURE',
-        )
-        props.context = 'TEXTURE'
+        if linestyle.active_texture:
+            slot = linestyle.texture_slots[linestyle.active_texture_index]
+            layout.template_ID(slot, "texture", new="texture.new")
+        else:
+            layout.template_ID(linestyle, "active_texture", new="texture.new")
 
 
 class VIEWLAYER_PT_freestyle_animation(ViewLayerFreestyleButtonsPanel, PropertiesAnimationMixin, Panel):