Add Shadowdark support

[fjallnari]

Meta-issue to keep track of Shadowdark (SD) support todos. Minimal system basics are already on add-shadowdark. Those first few are related to the sheet itself:

• Gear - similar to Cairn (slots and coins as paddles on side or as separate small boxes on top)
• Talents - same as gear, possibly a filter for talent origin
• Spells - spell level filter, spellcasting ability, spell-attack modifier, simple spells known table
• Attacks - should be linked with items (similar to how 5e works)
• Add Level-Up for Shadowdark #129
• Add Shadowdark SRD spells #131
• Add Shadowdark character generation #133

And the following should be in the GM's overview:

• Add Shadowdark turn tracker #130
• Add Shadowdark monsters view #132
• Add SD torch tracking #134

Misc stuff:

• JSON import - when the character schema is finalised, add SD JSON schema validation

[fjallnari]

Basic character sheet was added in #128.