Close #14 Make font customizable. Also manage user settings via a settings.json

file. Also improve 'dark' and 'elegant_dark' themes.
Finally improve 'Signal's wiki' button behaviour.
Also fix a bug in forecast/now view which caused a crash if solar activity
was inactive

src/settings.py

@@ -0,0 +1,44 @@
+import os.path
+from constants import Constants
+import json
+
+
+class Settings:
+    """Dynamically save and load the settings of the application."""
+
+    def __init__(self):
+        self._dct = {}
+
+    def load(self):
+        """Load the setiings.json file."""
+        if not os.path.exists(Constants.SETTINGS_FILE):
+            return
+        try:
+            with open(Constants.SETTINGS_FILE, 'r') as settings_file:
+                self._dct = json.load(settings_file)
+        except Exception:
+            pass  # Invalid file.
+
+    def save(self, **kwargs):
+        """Save the settings.json file.
+
+        Also update the current settings specified in kwargs.
+        New settings can be dynamically added via this method."""
+        for k, v in kwargs.items():
+            self._dct[k] = v
+        with open(Constants.SETTINGS_FILE, mode='w') as settings_file:
+            json.dump(
+                self._dct,
+                settings_file,
+                sort_keys=True,
+                indent=4
+            )
+
+    def __getattr__(self, attr):
+        """Return the corresponding setting.
+
+        Return None if there is no such setting yet."""
+        try:
+            return self._dct[attr]
+        except Exception:
+            return None