Methods for taking/detecting element focus

[jonhoo]

It would be very useful to be able to force focus to specific element (a text field for example), and similarly, to detect when an element gains focus. Something along the lines of:

textfield := ui.NewTextField()
textfield.OnFocus(func() {
    textfield.SetText("")
})
// ...
window.Show()
textfield.Focus()

If we implemented named events, OnFocus could be generalized to something like:

textfield.On(FOCUS, func(e ui.Event) { /* ... */ })

Where FOCUS is an enum of possible event types (well, technically a const of iota, but you get the point). This would also allow a uniform interface to similar operations such as OnChanged (similar to how event handling is done in JavaScript):

textfield.On(FOCUS, func(e ui.Event) { /* ... */ })
textfield.On(CHANGED, func(e ui.Event) { /* ... */ })
textfield.On(KEYDOWN, func(e ui.Event) { /* ... */ })
// etc..

[andlabs]

Partial merge with andlabs/libui#325. There's another issue I want to merge this with as well.

[andlabs]

Partial merge with andlabs/libui#326. This should handle all the needed merges; closing.