Refactor icons as a new Resource

[rsubtil]

Instead of offering core nodes that automatically load a controller icon based on a path, it might be possible to, instead, offer a new Resource type, inheriting from Texture2D, that can handle all the current functionality. This is a pretty cool idea because:

• Simplifies coding to a single implementation, instead of duplicated code that's currently on all 4 node types
• Allows more advanced use-cases and the possibility for developers to integrate such icons on custom nodes
• Does not require a script to be attached to the node for the icon to work
• Does not incur on any scene changes, which can currently happen when developers switch from keyboard to controller
• Optimization can be done more transparently to the end user (related to Optimize texture access on export #60)
• Due to raw drawing commands, it can easily support advanced use-cases, such as multi-icon drawing (e.g. XXY - Combo Attack), and embedded in other nodes such as RichTextLabel (e.g. Press F to pay respects)

Even with this system, there are some restrictions about usability that cannot change:

• The system must still work with plain image files like it currently does. Users need to be able to switch icon assets without additional steps.
• The current system to accept input actions, joypad paths, and icon paths must remain unchanged in behavior to the current system.

Plus, this will result in a new breaking change, and a very substantial one, requiring either automated or manual refactoring.

[rsubtil]

Another caveat is that queuing a redraw (necessary if input method changes) is not as easy. So far the best solution is to use Resource's emit_changed, but some Godot nodes do not listen to this, notably RichTextLabel. This will be reported as a bug and hopefully be fixed.

[rsubtil]

Update: The bug was reported and there's already a fix, so this won't block implementation. Will need to keep a note for any users using an older version of Godot when the fix is deployed, but other than that, this approach looks good.