[MU4 Issue] Implement amazon-style submenu triangle

[ecstrema]

Describe the bug
Navigating large submenus is a well-known, and resolved, design problem, and Qt's default behaviour tackles the problem fairly well.

The usual approach for more forgiving submenus was made famous by amazon's huge dropdowns and is very good. See this demonstration and explanation.

However, musescore's custom dropdown menus don't implement it for now, which is not a problem for most of musescore's menu right now, but is a pain for Add->all submenus and a few others like View->Toolbars.

To Reproduce
Steps to reproduce the behaviour:

1. Click on "Add" (top left)
2. Hover "Frames"
3. drag your mouse directly toward "append text frame"
4. The dropdown disappears and the "lines" or "text" dropdown appears instead.

Expected behaviour
Unless the movement clearly goes down and not right, keep the same menu, using amazon's strategy or similar.

Screenshots
Amazon's triangle

Platform information

• OS: Windows, probably all

[Tantacrul]

This is a pretty good idea. I don't see why not, although it might be some time before we get around to designing it. If a community dev wanted to try take it on, we could definitely provide design / testing support.