Ability to select multiple entities in the Entity Editor

[lodyb]

When selecting entities it would be good if multiple can be worked on at once

This could be done using shift select over a range (hold shift, click first, click last.) In windows it is also common to allow ctrl+click on extra items to add to the array of selected items.

Some ideas:

• A button to select every entity in the current filter
• A button to delete entities in the current selection
• Display the values from all selected entities as one
  • If every entity in selection has the same value, display that value
  • If there are different values in the selection for this key, display (different)
    • This is how Hammer handles the same behaviour so it will make sense to mappers

[tsa96]

This should be nice to do, we just create a temporary EntityViewModel when multiple are selected, without adding to the underlying cache in EntityLumpViewModel. Populate with properties shared by the selected entities, with all the behaviour lody suggested and display on the entity editor page, then observe on changes to their values and modify the viewmodels of the selected entities. HOPEFULLY, if I've structured everything well, we just need to add new stuff to the entity editor page, and shared viewmodels won't need changing at all.