fix: do not append text each time a widgets is opened

[TheLeoP]

Fixes #1042.

This is a really naive approach at solving the problem and I'm not sure if this somehow affects other places in the codebase. Since layer.render is being called without a parameter for lnum and end_ inside each widget, the render function appends the widget content instead of replacing it (wich does happend when the widget ir refresh. I saw that in that call 0 and -1 were being passed as values for lnum and end_, so I tried doing the same here and it worked).

[mfussenegger]

Thanks. I tried it for a bit and seems like there's no unintended side effect