Query: simplify concrete query declarations

Changes in https://github.com/sdroege/gstreamer-rs/pull/82 allowed using concrete types for queries directly, with this impact on attribute declaration:

Before:

    pub struct MyContext {
        [...]
        position_query: gst::Query,
    }

After:

    pub struct MyContext {
        [...]
        position_query: gst::query::Position<gst::Query>,
    }

This commit proposes to simplify the declaration to this:

    pub struct MyContext {
        [...]
        position_query: gst::query::Position,
    }

This way, the user doesn't have to be aware of internal details.