api: Fix various crashes on bad settings data. (#186)

getSetting now returns a straight nlohmann::json instead of an optional.
If the data isn't present, it will return a json null.

All accesses to the settings will first check that the data has the
expected type.

source/views/view_data_processor.cpp

@@ -20,8 +20,8 @@ namespace hex {
         View::subscribeEvent(Events::SettingsChanged, [](auto) {
             auto theme = ContentRegistry::Settings::getSetting("hex.builtin.setting.interface", "hex.builtin.setting.interface.color");
 
-            if (theme.has_value()) {
-                switch (static_cast<int>(theme.value())) {
+            if (theme.is_number()) {
+                switch (static_cast<int>(theme)) {
                     default:
                     case 0: /* Dark theme */
                         imnodes::StyleColorsDark();