Implement a tooltip feature, drawing a customizable element outside of the standard tree of elements

[TangentFoxy]

Inspired by the fact that I had to implement and position my own such feature for SCP Clicker. It'd be nice if the library handled positioning and potential problems automatically, but exposed the internals of the tooltip feature to customization.

Basically, the tooltip will check whatever is pop.hovered every frame, and only when something exists, create and move itself to the appropriate position. When pop.hovered does not contain a tooltip data item, it does nothing.

Not quite sure how I will allow customization, that will have to be implemented after I make something that is customizing it. (I imagine it is as simple as accessing pop.tooltip.data and modifying it.)

[TangentFoxy]

Note that a tooltip will always linger as currently implemented if the item is destroyed beneath it and it is not moved, as it is only listening to mousemoved events.

[TangentFoxy]

See Guard13007/SCP-Clicker#2

[TangentFoxy]

Relevant notes from linked issue:

• Note: Icons need to pass wheelmoved events to the box containing them! Make the add_icon function set this handler on them. Also the tooltip needs its own update separate from everything else I believe, in order to keep accidental lingering boxes around. Not sure.
• Tooltip will always linger if you click something and then don't move the mouse, as it is only updated in mousemoved if I remember correctly. A proper implementation will also have an update to check that it is still over something.
• In order for this to work, an element needs to be able to take over rendering its children, which is a feature I know I'd need at some point for smooth-scrolling areas anyhow.
• For now, my solution is to turn off the draw flag on all children of icon_grid. And then it will position and draw as needed.
• Actually, I can't do that, it would break mouse handling completely.