Replace KoToolButton with KoToolAction

Summary:

• change KoToolBox to use own buttons, made from new KoToolBoxButton which wires up to the KoToolAction
• change KoModeBox to directly use KoToolAction instead of the formerly used hack with the hidden QToolButton object of KoToolButton
• move creation of tooltip with added shortcut from ToolHelper to KoToolBox
• fixes new shortcuts having not been forwarded to any instances of KoToolBox, only the latest created
• fixes KoModeBox showing tooltips (with shortcuts) as tab labels

Differential Revision: https://phabricator.kde.org/D269