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