feat: module-level name and class options

BREAKING CHANGE: To allow for the `name` property, any widgets that were previously targeted by name should be targeted by class instead. This affects **all modules and all popups**, as well as several widgets inside modules. **This will break a lot of rules in your stylesheet**. To attempt to mitigate the damage, a migration script can be found [here](https://raw.githubusercontent.com/JakeStanger/ironbar/master/scripts/migrate-styles.sh) that should get you most of the way.

Resolves #75.

src/gtk_helpers.rs

@@ -0,0 +1,8 @@
+use glib::IsA;
+use gtk::prelude::*;
+use gtk::Widget;
+
+/// Adds a new CSS class to a widget.
+pub fn add_class<W: IsA<Widget>>(widget: &W, class: &str) {
+    widget.style_context().add_class(class);
+}