Migrate -dbcache setting from QSettings to settings.json

This is just the first of multiple settings that will be stored in the bitcoin
persistent setting file and shared with bitcoind, instead of being stored as Qt
settings backed by the windows registry or platform specific config files which
are ignored by bitcoind.

Co-Authored-By: furszy <matiasfurszyfer@protonmail.com>

src/qt/bitcoin.h

@@ -47,7 +47,7 @@ public:
     /// parameter interaction/setup based on rules
     void parameterSetup();
     /// Create options model
-    void createOptionsModel(bool resetSettings);
+    [[nodiscard]] bool createOptionsModel(bool resetSettings);
     /// Initialize prune setting
     void InitPruneSetting(int64_t prune_MiB);
     /// Create main window