Add snippets to source files and consoles

[jtpio]

For now snippets only work for notebooks, and a new cell is created when a snippet is chosen.

But it could actually be used for files and consoles too, by injecting the code snippet into the code editor at the current cursor location.

[sntgluca]

+1 for this request. Having the chance to inject code into a python script would be another step towards making Jupyterlab an unique IDE.