Hide objects and triggers

[chreden]

User should be able to hide objects and triggers.

• - In the objects and triggers windows there should be a new column to show that the object is hidden. This can be a checkbox so the user can hide it this way. The checkbox should be disabled by default - so it is hidden when it is checked.
• - The user should also be able to hide something by an entry in the right-click context menu.
• - There should be a view option to unhide all hidden objects.
• - Hidden objects should still appear in the appropriate lists in their windows, but just not in the 3D view.
• - If a hidden object is selected in the list it should still be rendered in the 3D view.
• - If an item is hidden it doesn't participate in picking.
• - Add tests for all touched areas of code

Initially planned, removed:

• - Pressing H when an object is selected in the viewer or in a window should toggle the hidden-ness of the object.
• - There should be a view mode to toggle showing of hidden objects.

Idea from @makotocchi

[chreden]

The work for rooms will be done as part of the rooms window (#517)