... deleting the last spot color.

When overprinting simulation is enabled while deleting the last spot color,
the map editor will become empty, and the overprinting action becomes
disabled. However, the program didn't actually activate the overprinting
simulation mode, so the user was locked out from re-enabling the normal
rendering of the map. We must explicitly trigger the action.

(Reactions are bound to trigger() to avoid hard-to-debug activation loops
from normal state changes.)

Co-authored-by: Libor Pechacek <lpechacek@gmx.com>

A color entry may serve as a spot color without direct use in symbols.
So far Mapper allowed silent removal of these color entries. This patch
adds a warning which is shown when the user tries to remove such a spot
color.

Co-authored-by: Matthias Kuehlewein <github@kuehlewein.de>
Co-authored-by: Kai Pastor <dg0yt@darc.de>

Add function to return a list of map colors that use a specific spot
color.
Remove function that just counts these map colors.

Merging the functionality of issuing a warning about spot color removal
with the functionality of counting the usage in symbols and objects.
As a result just a single warning is showed. Use 'setDetailedText'
function to show more details with respect to spot color removal.