allow to set custom icon for the left drawer handle

Summary:
it allows to set a icon to the globaldrawer handle, for that both close and open handle icons need to be set

FEATURE: 405598
FIXED-IN: 5.62

Reviewers: #kirigami, mart

Reviewed By: #kirigami, mart

Subscribers: ngraham, plasma-devel

Tags: #kirigami

Differential Revision: https://phabricator.kde.org/D23154