GH-4 | Migrate API calls to `callApi`

[mdziekon]

Closes #4

Changelog:

• [x] API calls migration to callApi (and add basic error handling to call site)
  • [x] callAdditionalSettings (removed)
  • [x] loadDatabaseMetaData
  • [x] testDatabaseConnection
  • [x] confirmDatabaseProblemMessage
  • [x] callLoadSpoolsByQuery
  • [x] callSaveSpool (already done)
  • [x] callDeleteSpool (already done)
  • [x] callSelectSpool
  • [x] allowedToPrint
  • [x] startPrintConfirmed
  • [x] callDeleteDatabase
• [x] Investigate _buildRequestQuery simplification
• [x] callApi should better handle error responses
• [x] callApi should automatically detect "contentful" requests and attach appropriate headers