Refactor filenames

[ghost]

Since the number of files and functions have increased considerably, I decided that it would be a good to refactor the code at this point.

I have not added, removed, or changed any features.

In order to make the whole thing easier to grasp

1) Modified the file names.

2) Moved related methods from one file to another based on receiver, as possible, but not all.

Added) Ah! I made a mistake in git mv. There's something wrong with focusable.v, layout.v, and widget.v, but there's nothing wrong with the content.

[ghost]

This is a bug of v fmt.

This works

        if widget.id !in em.receivers[evt_type].map(it.id) {
            em.receivers[events.on_mouse_down] << widget
            $if evt_mngr ? {
                println('add receiver $widget.id for $evt_type')
            }
        }

But v fmt inserts . before events.on_mouse_down like .events.on_mouse_down.

        if widget.id !in em.receivers[evt_type].map(it.id) {
            em.receivers[.events.on_mouse_down] << widget
            $if evt_mngr ? {
                println('add receiver $widget.id for $evt_type')
            }
        }