Minor string improvements

Use sentence capitalisation for tooltips and header capitalisation for
menu items. Only use an ellipsis when the action required further user
interaction, as per the GNOME HIG:

http://developer.gnome.org/hig-book/3.7/design-text-labels.html.en#layout-capitalization
http://developer.gnome.org/hig-book/3.7/menus-design.html.en#menu-item-type-command