TE: Add grouping of objects

[Nickelony]

Like how groups are done in Hammer Editor, my idea is to have a new dockable window which displays a list of groups, where next to each group record, there will be a checkbox which will hide the given object group, preventing it from compiling into the final level.

Additionally, we can allow groups to be used to quickly select multiple objects, without having to always re-select them each time.

Example from Hammer Editor: