add a container for styling constants, so we don't repeat literals all over the code

electrum/gui/qml/components/Constants.qml

@@ -0,0 +1,16 @@
+import QtQuick 2.6
+import QtQuick.Controls.Material 2.0
+
+QtObject {
+    readonly property int paddingSmall: 8
+    readonly property int paddingMedium: 12
+    readonly property int paddingLarge: 16
+    readonly property int paddingXLarge: 20
+
+    readonly property int fontSizeXSmall: 10
+    readonly property int fontSizeSmall: 12
+    readonly property int fontSizeMedium: 15
+    readonly property int fontSizeLarge: 18
+    readonly property int fontSizeXLarge: 22
+    readonly property int fontSizeXXLarge: 28
+}

electrum/gui/qml/components/main.qml

@@ -19,6 +19,8 @@ ApplicationWindow
     Material.primary: Material.Indigo
     Material.accent: Material.LightBlue
 
+    property QtObject constants: Constants {}
+
     property alias stack: mainStackView
 
     header: ToolBar {