Enable paste support in views.Application

[chrBrd]

Fixes #552

Identical to c3ed795, but for pasting.

[gdamore]

I'm less certain I like this ... although I guess it means applications will need to prepared for events they don't recognize (paste start and end).

In dcell ( a new D implementation of this) I made paste a single event, and would definitely think it should be opt-in.

[chrBrd]

The implementation in this PR is the least 'invasive' way of enabling pasting for an Application, but I'd agree the functionality would ideally be opt-in (I'm inclined to think mouse support should be too, but that would break BC now).

An alternatve approach that would allow opt-in could be something like:

type Application struct {
	widget Widget
	...
	paste  bool
}

func (app *Application) EnablePaste(on bool) {
	app.paste = on
}

func (app *Application) run() {
	...
	if app.paste {
		screen.EnablePaste()
	}
	...
}

I'm happy to implement it if you're fine with the approach?

[chrBrd]

PR updated - I've gone ahead and implemented the changes I proposed above, should hopefully result in less 'back-and-forth' if you're happy with the solution. :)

[gdamore]

Thanks for your contribution! If you want a listing in AUTHORS please submit a follow up PR.