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.
Add a context menu to the toolbar to change the position of the text

Closes #1145.
Merge pull request #2046 from pmzqla/toolbar-menu

Allow minor toolbar customizations