add latex input layer to process input only from this layer

[tbobek]

create a layer 'Latex Input' during the first call of the extension, using latex input from all layers (like before)
The next calls only use the 'Latex Input' layer for latex-ing.

[seebk]

Thanks for your pull request. I like the idea to only read text input from a specific layer. There is already another PR #13 that implements a similar feature. It lists all layers in a dropdown list to allow the selection of a ninput layer. As it is a bit more advanced and convenient I would prefer to merge this solution instead of yours. However, no estimate yet when it will done. Anyway, thanks again for your effort!