Redesign if-branches for clarity

This prepares handling non-unit entities for #1572
Move folder content/gui/icons/{units => }/thumbnails

Nobody claims this is only allowed to contain unit thumbnails.
Display unknown units in multi-select tab
Pass selected instances to multi-select tab instead of session

Not sure this really is a more reasonable way to handle things,
requiring access to instance.session access feels pretty bad.
Condense multi-select hide logic
Show non-unit entities in multi-select tab

Fixes #1572 - Show something when multiple buildings are selected