* feat: apply margin to `osd-margin-y` and `osd-margin-x`
The margins were already set for `osc-margins` and
`user-data/osc/margins`, but osd messages have their own properties
`osd-margin-x` and `osd-margin-y`.
Offsetting `osd-margin-x` and `osd-margin-y` by our margin prevents
overlap of osd messages with the GUI.
`mbtn_left` and `wheel` events are now only fired on a `cursor.on_{event}` object, which resets at the beginning of each frame, so elements need to bind these listeners in their render functions. These properties are overwritable which allows elements to take over cursor event handling from their parents if necessary.
Added option `top_bar_alt_title`, which accepts an alternative title template to be displayed, and `top_bar_alt_title_place` that controls where it should be displayed. It can be either `below` to place it below the main one, or `toggle` to show it in place of the main title when user clicks the top bar or calls the `toggle-title` binding.
The `top_bar_title` now also accepts a custom top bar title template if user wants it to be different from the one defined in `mpv.conf`.
closes#402
