implement modern sqlite support (e.g. windows, aggregates)

[machow]

tbl_mtcars >> filter(_.mpg > _.mpg.mean())

[machow]

Note that in general sqlite has a good support for afaik windows and aggregates these days, so changing issue to focus on that

[machow]

Support for window functions was added 4 years ago, so it is likely safe to just add everything into the current sqlite dialect (note that dbplyr checks the sqlite version before selecting the right sqlite dialect)

https://www.sqlite.org/changes.html

[machow]

Resolved by #406