Misc elements issues

[djowel]

• [x] Layout example: menu not processing up/down arrows correctly.
  • [x] Add cast utility that deals with indirect elements.
• [ ] Elements ‘image’ should apply the _scale since artist image does not have a scale parameter anymore (in artist)
• [x] Test sprites! — should there be an option not to anti-alias images? Should the scale always be 1:1 so there are no scaling artifacts?
• [x] Popups example does not respond to return key unless you click the alert first
• [x] There are no left_right_margin and top_bottom_margin
• [ ] Elements with callbacks such as on_click should be ’sender’s (see basic_choice for example)
• [x] Warnings as errors
• [x] Add this to elements docs: https://www.cycfi.com/2019/07/photon-micro-gui/
• [ ] Fix windows quirk when moving from a non-high-dpi window to a high-dpi window
• [x] Add the margin API variants. See http://cycfi.github.io/elements/layout#vmargin (Related: https://github.com/cycfi/elements/issues/145)
• [ ] Use noexcept wherever appropriate
• [x] Consider returning bool in click member function.
• [ ] Apply the stretch attribute to aligns. Related to 208

Low Priority:

• [ ] Add an MVCE for #215 for the Cairo devs.

Each of these may move to an actual issue when needed (e.g. if it's not an easy fix).

[Xeverous]

Shouldn't this issue be pinned?

[djowel]

Shouldn't this issue be pinned?

Done.