Open itinerary menu on left click

Summary: Having to right click seems unintuitive to me. Also it is more touch-friendly this way. The old left-click action is still available in the menu

Reviewers: vkrause, #kde_pim_kmail

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim, #kde_itinerary

Differential Revision: https://phabricator.kde.org/D17823