Add read/save support for keyboard shortcuts.

- Store toolbar actions into action collection of class KXMLGUIClient,
  which is required to be listed in the shortcuts settings dialog.
  To be able to access tool bar actions with tbb_... constants, save
  tool bar actions in WorkToolBar::m_actions, which makes WorkToolBar::m_toolButtons
  and class ToolButton obsolate; tool bar related cursors are stored in
  WorkToolBar::m_cursors.
- Save changed shortcuts in [Shortcuts] section of umbrello settings file 'umbrellorc'
- Show tool bar button shortcut in button tooltip.

CCBUG:124804