One-click copy buttons

[jarruda]

• An egui button with a copy icon and default alt-text of "Copy".
• Button should be included in the layout of any copy-able element,
• Button should be hidden by default in the common case, and appear when the cursor is over the element to be copied (or some appropriate parent element, like a table row.

Example locations:

• Each log entry - copy message or copy entire raw entry.
• Context view - copy entire context object, or each value individually