Fix the description of "layer" param in Action::LayerSetDesc

"Layer to be moved" still is the description of Action::LayerMove
Enhancement #335 - UI : Renaming a value node, display old name

Add value_provided to ParamDesc
Enhancement #335 - UI : Renaming a value node, display old name

Add virtual bool Action::Base::get_param { return false; }
Add virtual bool Action::CanvasSpecific::get_param { return false; }
Enhancement #335 - UI : Renaming a value node, display old name

Add set_value_provided() for name parameter
Add Action::ValueNodeRename::get_param
Enhancement #335 - UI : Renaming a value node, display old name

In Instance::process_action , is the param is user_supplied,
check if it's also value_provided, and get it.
Enhancement #335 - UI : Renaming a value node, display old name

Give same behavior to Action::LayerSetDesc (current name displayed in dialog)

- set_value_provided() for new_description param
- implement Action::LayerSetDesc::get_param
Merge pull request #102 from d-j-a-y/djay_action_param_get_value

Display old name when renaming an exported value (closes #335)