Use the default fallback behavior of localforage

Since we store a compressed base64 string we can use indexeddb on mobile when available.

This PR removes the the use of websql by default.

Tested with a 15km2 extent with 3 layers (pixelkarte, wander, cadastralwebmap), here the result:

Amdroid Chrome: 99mo,
Amdroid  Firefox: 71mo
Amdroid Opera: 138mo

ios 8 Safari: 58 mo