Stylesheet : Style MenuBar extension button

[ericmehl]

This makes the menu extension button more visible. That button is shown when the window is too narrow for the full menu bar. Qt's default is a pair or pretty dark arrows / chevrons, which are not very visible against Gaffer's dark menu background.

Checklist

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have tested my change(s) in the test suite, and added new test cases where necessary.
• My code follows the Gaffer project's prevailing coding style and conventions.

[johnhaddon]

Thanks Eric. I rebased to account for the recent release in Changes.md - merging.