Composer: Use popover for Link Properties
Evolution now uses GtkPopover for the majority of insertion actions from the composer's toolbar (such as inserting emojis, changing colors, etc.), but the hyperlink insertion (or editing) dialog is still an old modal dialog on top of the composer:
Switching to use a GtkPopover positioned at the selection/text cursor's position would be more visually consistent, more contextual¹, and take less space (it could be simplified so that no X/Close buttons are even necessary: clicking outside the popover cancels/dismisses the changes). Geary uses that approach, and it feels much nicer in terms of UX.
This is what it looks like when creating a hyperlink in Geary (note that it requires you to have selected text to hyperlink prior to making the toolbar button sensitive; as such, it can have a simpler UI since it doesn't have a "description" field to set the text, as it is not possible to insert a hyperlink with custom text at the cursor position, with no selected text):
...and this is how it looks like when the cursor is within an existing hyperlinked text and you click the button, to modify, update or remove the hyperlink:
¹: In an ideal world I would imagine it would be nicer to have the popover appear from the position of the text selection/cursor rather than from the toolbar (same goes for emoji insertion popover), but I don't know if that's possible...