Sensible default widgets

[germangb]

At the moment, the default widget is a label, which means that if you don't specify a widget type inside an annotation, that particular struct field is rendered as a text label (provided it implements Display), which is not interactive.

The idea is to define the following default widgets for each type:

rust type	default widget	remarks
i32, u32, f32, [f32; 2], (f32, f32) etc...	input
bool	checkbox
String, ImString	input	String might need an extra allocation, which is not desirable

NOTE: Table not definitive

This way you can save a few keystrokes:

#[derive(imgui_ext::Gui)]
struct Example {
    #[imgui(checkbox)]
    a: bool,
    #[imgui(input)]
    b: [f32; 2],
}

// would become equivalent to:

#[derive(imgui_ext::Gui)]
struct Example {
    #[imgui]
    a: bool,
    #[imgui]
    b: [f32; 2],
}