When dragging an email, GUI cues (such as which label will receive the drop) are missing

[panicsteve]

Prerequisites

• [X] I have searched open and closed issues for duplicates.

Describe the bug

Two issues related to dragging an email that I found jarring, coming from macOS:

1. While dragging an email, there is no icon or other visual representation of what is being dragged attached to the mouse pointer. I would expect to see a small envelope icon or something similar, attached to the mouse pointer for the duration of the drag.

When the ability to drag multiple messages at once is added (#322), it would be nice if this icon could be badged with the number of messages being dragged.

2. While the drag is in progress, as the mouse hovers over valid drop targets (such as labels in the sidebar) there should be some visual indication that the target can and will accept the drop if the mouse button is released. For example, the label could be drawn in a highlighted style while an acceptable drag is hovering over it.

One thing that does change when the drag is hovering over a label is the mouse pointer, which changes from a closed hand to a closed hand with a 4-way directional arrow. This doesn't really correlate to the action that will be performed, IMO.

To Reproduce

Steps to reproduce the behavior:

1. Use the mouse to drag an email to a label in the sidebar.

Expected behavior

See description above.

Screenshots or screen recordings

Will provide if desired.

Logs

Will provide if desired.

Platform Information

• [ ] I'm using the latest version from git that I've manually compiled
• [X] I'm using the latest released stable version

Additional context