Rework HBox-based pages and account settings

- Refactor everything about HBox, and adapt all the pages and popups
  that used it

- Replace HTabContainer by HTabbedBox

- Make boxes swippable

- Make esc presses in boxes click the cancel button

- Make all boxes and popups scrollable when needed

- Replace generic apply button icons in popups

- Fix tab focus for error and invite popups

- Rework (still WIP) the account settings page:
  - Use the standard tabbed design of other pages
  - Ditch the horizontal profile layout, hacky and impossible to extend
  - Add real-time coloring for the display name field

- Implement a device list in account settings (Sessions, still WIP)

src/gui/Base/HPopup.qml

@@ -6,12 +6,15 @@ import CppUtils 0.1
 
 Popup {
     id: popup
-    anchors.centerIn: Overlay.overlay
     modal: true
     focus: true
     padding: 0
     margins: theme.spacing
 
+    // FIXME: Qt 5.15: `anchors.centerIn: Overlay.overlay` + transition broken
+    x: (parent.width - width) / 2
+    y: (parent.height - height) / 2
+
     enter: Transition {
         HNumberAnimation { property: "scale"; from: 0; to: 1; overshoot: 4 }
     }