Summon several kinds of kittycat from Bag of Cats

[wapcaplet]

Summary

SUMMARY: Features "Allow summoning spells to spawn from a monster group"

Purpose of change

To get all different kinds of cats from the Magiclysm "Bag of Cats" spell, which I've been wanting to do ever since #38889.

Describe the solution

Add a new flag spell_flag::SPAWN_GROUP, which can be provided in a spells "flags" list.

For spells with "effect": "summon", the flag causes the spell to treat the "effect_str" entry as a monster group ID, rather than a monster ID.

In other words, summoning spells used to be limited to a single monster type:

"effect": "summon",
"effect_str": "mon_cat"

Now, they can summon a monster from a group, like so:

"effect": "summon",
"effect_str": "GROUP_STRAY_CATS",
"flags": [ "SPAWN_GROUP" ]

The new flag is documented in a new "Spell Flags" section of MAGIC.md.

Describe alternatives you've considered

I poked at some dead-ends with an effect_list and hard-coded random monster spawns before @KorGgenT pointed me toward this more reasonable solution.

The SPAWN_GROUP flag may be useful for the item-spawning (and possibly vehicle-spawning) spell effects as well, but I did not attempt to implement such for this PR.

Testing

Casting "Bag of Cats" gave a variety of feline forms, as intended.

Tested "Cause Bear" and "Summon Decayed Pouncer" to ensure original summoning behavior still works.

Additional context

There is still one problem I want to solve, which is the way the spell displays in the spell menu. Because monster group IDs do not really have player-readable names, and the spell description layout appears to have a maximum usable width for this field, the full name of the group will not fit:

This is fixed now in e5433e4 - see comment below.

[wapcaplet]

I found what I think is a good solution to the cutoff of the summoned monster name - effectively swapping the positions of the "Damage" and "Range" sections of the spell info display, so the "Damage" (or "Summon") section gets its own line, and "Range" shares the first line with "AOE".

Here, pictures. Before:

After:

And this is what "Bag of Cats" looks like now:

(Note, final terminology is "2 from GROUP" instead of "2 of GROUP")

[kwl01skz]

What surprised me was that so many cats were wandering around in my house without even breaking my can.

[wapcaplet]

Travis CI shows an error that appears unrelated to anything in this PR:

src/character.cpp:9477 [Character::migrate_items_to_storage(bool)::<lambda(const item*)>]
ERROR: Could not put telescoping umbrella into inventory.
Check if the profession has enough space.
...
Treating result as failure due to error logged during initialization.