Add texts to the icons on the toolbar

The toolbar follows the system style to display texts associated to
icons. Since no specific text is provided, the text used for the action
associated to the button will be used. This could make some of the
buttons rather big.

Add short texts to the icons so that each button uses a reasonable
amount of space.
Remove unneeded tooltip

A tooltip will be generated using the text of the action anyway.
Add context menu to allow minor toolbar customizations

Add a simple context menu to the toolbar to set the size of the icons
and the position of text.