Update gtk requirement from 0.7 to 0.8

Updates the requirements on gtk to permit the latest version.

Changelog

*Sourced from [gtk's changelog](https://github.com/gtk-rs/gtk/blob/master/CHANGELOG.md).* > ## 0.0.8 (master) > > ## 0.0.7 (16-Mar-2016) > > ### Breaking changes > > - [Object reform][gtk221] introduced autogenerated high-level bindings and > broke some conventions. > > - Constructors don't return `Option`, they never fail. > > - Companion traits renamed e.g. `ButtonTrait` -> `ButtonExt`. > > - Modules are private now, all items are exported at the crate level (e.g. > `gtk::button::Button` is no longer visible, use `gtk::Button` instead). > > - Dialog APIs made less Rust-y for consistency with upstream. > > - File name types [changed][gtk236] from `&str` to `T: AsRef`. > > - A few signatures and spellings corrected to match upstream. > > - Version features [reintroduced][gtk248] and autodetection is no more. The > fallback version is 3.4. > > Select the minimal version required by your application like this: > ```toml > [dependencies.gtk] > features = ["v3_10"] > ``` > > - Signal handlers [take the receiver by reference][gtk226-1] and support [more > specific receiver types][gtk226-2]. > > - New [safe event bindings][gdk95] are [passed to signal handlers][gtk244]. > These bindings have [safe getters][gdk107] while the low-level event structs are > still reachable [via `as_ref`][examples68]. > > - New safe [`Error`][glib86-error] and [`Value`][glib95] bindings. `ListStore` and > `TreeStore` [changed accordingly][gtk238], traded `set_string` for more > powerful [`set`][gtk241]. > > - In an ongoing move to [reflect integer unsignedness properly][gtk246] > `Notebook` switched from `i32` to `u32`, `ListStore` and `TreeStore` partly > switched to `u32` and `EntryBuffer` switched to `u16`. > > ### Improvements > > - [Object reform][gtk221] benefits include: > > ... (truncated)

Commits

- [`7d29cea`](https://github.com/gtk-rs/gtk/commit/7d29cea5854fab3d2b318f5baa4ba354bd21c853) fix gtk-sys version - [`258706b`](https://github.com/gtk-rs/gtk/commit/258706bd509b0ad267b1d89205687f87d8089af4) Merge pull request [#925](https://github-redirect.dependabot.com/gtk-rs/gtk/issues/925) from gtk-rs/crate-release-update - [`0e618ff`](https://github.com/gtk-rs/gtk/commit/0e618ff7ed5d05a5739eec36f8ede6839f8c0eb7) Update versions [ci skip] - [`1e4d0e1`](https://github.com/gtk-rs/gtk/commit/1e4d0e1c45fec76c8476e5767cc6d7b23b86e718) Merge remote-tracking branch 'origin/master' into crate - [`0bd621f`](https://github.com/gtk-rs/gtk/commit/0bd621f4df73754c9043b252cdb4f8fa1cde31ad) Merge pull request [#923](https://github-redirect.dependabot.com/gtk-rs/gtk/issues/923) from GuillaumeGomez/regen - [`1754d17`](https://github.com/gtk-rs/gtk/commit/1754d17a0b44a91ce4fe6dd28f0614bc9b43c8ce) regen - [`25332a0`](https://github.com/gtk-rs/gtk/commit/25332a0d722deb143861a18b28429d62d9cd780d) Update gir submodule - [`2efce6e`](https://github.com/gtk-rs/gtk/commit/2efce6e8b8a8db34b87e1555e877b26e34d3d7cc) Merge pull request [#921](https://github-redirect.dependabot.com/gtk-rs/gtk/issues/921) from alatiera/alatiera/widget-parent-events - [`c38889e`](https://github.com/gtk-rs/gtk/commit/c38889e7f7169d637b863c68c6b30f0eaed653e6) subclass/window: widget in set_focus should be Nullable - [`ea45775`](https://github.com/gtk-rs/gtk/commit/ea45775630bbf006ac6c1450a15961da15cfb867) subclass/{widget, application, dialog, window, container}: Implement default ... - Additional commits viewable in [compare view](https://github.com/gtk-rs/gtk/compare/0.7.0...0.8.0)

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sanpii / effitask

Update gtk requirement from 0.7 to 0.8 #42