Compose is Unclear on macOS and in Testing Instructions

[cailafinn]

Describe the bug
On macOS the dropdown for the compose button is crushed such that it is easy to miss the ability to select Scale or Bose.

In addition, the manual testing instructions mention using the compose button but if Scale or Bose have already been selected previously, it can be difficult to find.

To Reproduce

1. Open Interfaces -> Direct -> MSlice
2. Switch to the Workspace Manager tab.

Expected behavior

Modifying the Compose button to behave similarly to the Save dropdown where selecting an option from the dropdown automatically starts the saving process.

It may also be worth updating the manual testing instructions to indicate that this button may read Scale or Bose.

Screenshots

Platform/Version (please complete the following information):

• OS: [e.g. Windows, RHEL 7, Ubuntu, macOS]
• OS Version:
• Mantid Version [e.g. 6.0.0]

Additional context