Add background image variable to theme

TODO.md

@@ -75,7 +75,6 @@
     - Find icon packs in user data dir
     - Correctly implement uiScale/fontScale + ctrl+-= keys
       - See [Text.fontSizeMode](https://doc.qt.io/qt-5/qml-qtquick-text.html#fontSizeMode-prop)
-    - Selecting background image
     - Way to round avatar corners to allow box radius
     - If avatar is set, name color from average color?
     - Accent color from background

src/qml/UI.qml

@@ -35,10 +35,10 @@ HRectangle {
         accounts.count > 0 || py.loadingAccounts
 
     HImage {
-        visible: false
+        visible: Boolean(Qt.resolvedUrl(source))
         id: mainUIBackground
         fillMode: Image.PreserveAspectCrop
-        source: "../images/background.jpg"
+        source: theme.ui.image
         sourceSize.width: Screen.width
         sourceSize.height: Screen.height
         anchors.fill: parent

src/themes/Default.qpl

@@ -2,6 +2,8 @@
 // This file is part of harmonyqml, licensed under LGPLv3.
 // vim: syntax=qml
 
+// Base variables
+
 real uiScale: 1.0  /* TODO: Implement correctly, do not change for now */
 real fontScale: uiScale
 
@@ -62,6 +64,8 @@ colors:
         loops: Animation.Infinite
 
 
+// Generic UI controls
+
 controls:
     box:
         color background: colors.strongBackground
@@ -120,7 +124,11 @@ controls:
         int lightness:  55
 
 
+// Special UI parts
+
 ui:
+    // Supported: local file path or web URL
+    url image: ""
     color background: colors.weakBackground
 
 
@@ -137,19 +145,19 @@ sidePane:
     color background: colors.strongBackground
 
     account:
-        color background: sidePane.background
+        color background: "transparent"
         color name:       colors.text
 
     room:
-        color background: sidePane.background
+        color background: "transparent"
         color name:       colors.text
         color subtitle:   colors.dimText
 
     settingsButton:
-        color background: sidePane.background
+        color background: "transparent"
 
     filterRooms:
-        color background: sidePane.background
+        color background: "transparent"
 
 
 chat: