Disable objectives that are already on the table when adding new public objectives

[tarnas14]

currently:

• user clicks on (1.) to add a new objective
• "Erect a Monument" is already on the table (3.) (added previously)
• user can choose "Erect a Monument" from dropdown (2.)

desired behaviour: public objectives that are already on the table should be shown on the dropdown list, but in a disabled state, with a tooltip / title text on hover with an explanation that "This objective is already in play"