Update the way id is generated to be used as variable name

[ecarriou]

For a System Designer example, I have created some Web Component-like widgets:

• widgets are generated with custom tags,
• widgets are in fact components and their DOM id are their internal id and
• in that case we can not use a CSS selector to select a widget by its id: its id is not DOM compliant.

=> we need to generate id DOM compliant, i.e. that can not begin with a number.