Desktop: Fixes #11274: Fix content dropped into the Markdown editor is missing a cursor preview or dropped at the wrong location

[personalizedrefrigerator]

Summary

This pull request:

1. resolves Dragging and dropping text no longer shows preview caret #11274
2. resolves a similar (but not identical) issue reported on the forum.

In particular:

1. In-editor content was dropped at the correct location, but no preview cursor was shown.
2. Notes were dropped at the last editor cursor location.

Note

This pull request targets release-3.1.

Screen recording

Screencast.from.2024-10-29.15-54-21.webm

Manual testing plan

MacOS (Sequoia): (MacOS testing done before rebasing on release-3.1).

1. Drag a note from the note list over the editor.
2. Verify that a preview cursor is visible.
3. Drop the note at a location distinct from the current editor cursor.
4. Verify that the note is dropped at that position.

Fedora 41:

1. Open Chromium.
   • Files from Firefox and the Files app often fail to drop into Joplin. This is the case with both with the new and legacy editors. Drag and drop from Chromium's "Downloads" list seems to work consistently.
2. Download 2-3 images (2 PNGs, 1 SVG).
3. Drop a PNG from the downloads list into Joplin's Markdown editor.
4. Verify that the image drops successfully.
5. Drop the SVG from the downloads list into a different location in Joplin's Markdown editor.
6. Verify that the image is dropped at the mouse's last location.
7. Drop a note from the note list into the editor.
8. Verify that a note link is added at the mouse's last location.
9. Select text in the editor.
10. Drag and drop the text to a different part of the editor.
11. Verify that the text is moved.

[personalizedrefrigerator]

This change shows where content will be dropped. See the CodeMirror documentation for dropCursor.

[personalizedrefrigerator]

posAtCoords is how CodeMirror internally determines the drop location for content — this makes the drop location of content processed by Joplin match CodeMirror's drop cursor.