Add a persistent zoom setting to settings.json

The zoom keybinds now modify this setting directly.
The `uiScale` property in themes now default to using this zoom setting,
since just removing it would need of annoying changes through theme
files and QML code, and is best left to until the future theming
overhaul update.

src/gui/UI.qml

@@ -64,17 +64,26 @@ Item {
 
     HShortcut {
         sequences: window.settings.keys.zoomIn
-        onActivated: theme.uiScale += 0.1
+        onActivated: {
+            window.settings.zoom += 0.1
+            window.settingsChanged()
+        }
     }
 
     HShortcut {
         sequences: window.settings.keys.zoomOut
-        onActivated: theme.uiScale = Math.max(0.1, theme.uiScale - 0.1)
+        onActivated: {
+            window.settings.zoom = Math.max(0.1, window.settings.zoom - 0.1)
+            window.settingsChanged()
+        }
     }
 
     HShortcut {
         sequences: window.settings.keys.zoomReset
-        onActivated: theme.uiScale = 1
+        onActivated: {
+            window.settings.zoom = 1
+            window.settingsChanged()
+        }
     }
 
     HShortcut {