Misc elements issues

[djowel]

• Layout example: menu not processing up/down arrows correctly.
  • 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)
• 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?
• Popups example does not respond to return key unless you click the alert first
• 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)
• Warnings as errors
• 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
• Add the margin API variants. See http://cycfi.github.io/elements/layout#vmargin
  (Related: margin/align name inconsistency #145)
• Use noexcept wherever appropriate
• Consider returning bool in click member function.
• Apply the stretch attribute to aligns. Related to 208

Low Priority:

• Add an MVCE for Cropped characters in text_edit sample  #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.