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
Simplify cursor reset in multi-selection
Convert a loop to any()
Rename variable for clarity

This can now be a building with attacks as well.
Allow multiselect combinations of units and buildings

Both types can share certain commands (attack, destroy) so it does not
impact code nor interface sanity to allow this, even if the comment
implied otherwise.
Merge pull request #2009 from unknown-horizons/multiselect

Include buildings in multi-selection tab for #1572