GTK double click handling causes Stepper to skip values.

Nobody has claimed this yet.

The Stepper, despite being (currently, see #762 ) exclusively for floats, does not seem to implement clamping against rounding errors.

Given the following compilable example (with warnings):

```rs
use druid::{AppLauncher, WindowDesc, Widget, PlatformError, LensWrap, Lens, Data, LensExt};
use druid::widget::{
Padding, Stepper, Switch, WidgetExt, Flex, Label, TextBox, Parse
};
fn main() {
AppLauncher::with_window(WindowDesc::new(build_ui)).launch(State {
n_value: 20.0
});
}
fn build_ui() -> impl Widget {
let mut row = Flex::row();
let n_textbox = LensWrap::new(
Parse::new(TextBox::new()),
State::n_value.map(|x| Some(*x), |x, y| *x = y.unwrap_or(0.0)),
);
let stepper = LensWrap::new(
Stepper::new()
.with_range(0.0, u32::MAX.into())
.with_step(1.0)
.with_wraparound(false),
State::n_value,
);
row.add_child(Padding::new(5.0, n_textbox));
row.add_child(Padding::new(5.0, stepper));
row.center()
}

#[derive(Lens, Data, Clone)]
struct State {
n_value: f64,
}
```

Use the buttons to go up and down a few times, and you'll note that some values will end up being skipped.

As a note, I'm building off of 0.6.0, and would state that building off of master is something that should be discouraged (so if this is fixed in master, I request this not be closed until the release is pushed onto crates.io)

Start by reproducing the skipped values with the compilable Rust example in the issue, then inspect the Stepper implementation and its handling of floating-point ranges, steps, and button events. Done means repeated upward and downward clicks no longer skip values within the configured range; add or run a regression test if the existing test structure supports it.

Written by the indexing model from the issue text.