Customize ContextDrawer handle icon

Summary:
Use more contextually-appropriate icon for Kamoso's right ContextDrawer.
We can't yet do the same thing for the left GlobalDrawer due to a Kirigami bug:
https://bugs.kde.org/show_bug.cgi?id=405598

Test Plan: {F6700109}

Reviewers: apol, #vdg

Reviewed By: apol

Differential Revision: https://phabricator.kde.org/D19853