Native style support for Qt Quick Controls 2

This work document describes the ongoing work to port QStyle to QtQuick Controls 2, with the overall goal of improving support for writing desktop applications with a native Look-And-Feel.

Controls available, but missing native style (supported by QStyle)

• ToolButton (Button with menu)
• ToolSeparator

Controls available, but missing native style (unsupported by QStyle)

• Switch
• RangeSlider
• BusyIndicator
• DelayButton
• PageIndicator
• Tumbler

Controls missing (compared to Qt Widgets)

• TabView
  • cannot be composed as TabBar + StackView, it's has more combined visuals on e.g macOS
• ScrollableTextArea (same concept as on macOS)
  • or improve support putting TextArea inside ScrollView
• QToolBar
  • ToolBar (and side bar) should have a combined visual look on macOS
• QFontComboBox
• QTimeEdit
• QDateEdit
• QKeySequenceEdit
• QMdiArea
• QMdiSubWindow
• QDockWidget
• QToolBox
• QCommandLinkButton
• Line (in Qt Designer, which is a Frame. Integrate into ToolSeparator?)

Controls missing (compared to XCode IB)

• Segmented button (mac) in controls1 as ButtonGroup/ButtonRow?
• SearchField 

 Controls available, but with missing API (compared to Qt Widgets)

• Small and Mini API for most controls
• TextArea
  • BorderStyle. Important to be able to place it inside e.g a Frame or a ScrollView that already has a frame.
  • Putting it inside a ScrollView should just work
    • suffers from the same problems as with flickable above.
  • frameStyle (Shape, Shadow)
  • frameShape (Box, Panel, WinPanel, etc)
    • or perhaps borderStyle (border is more QML): (none, Line, bezel, rounded)
      sborder style/frameShape (none, Line, bezel, rounded)
• ScrollBar
  • Add API to specify if a scrollbar should be transient or not (fade away when not used). Currently a scrollbar is transient if the the policy is ScrollBar.AsNeeded. But this is different for a desktop style where the scrollbar should typically stay or hide depending on the content/viewport ratio, and not if it's being actively used. This should also control if the scrollbars are used as overlays in ScrollArea, or placed on the side of the contents.
  • On mac, work is needed to animate transient scrollbars.
  • On mac, the background should sometimes have a frame, other times not. But it looks like the stand-alone version should not have any frames. But when used in a NSScrollView, it should. So investigate more how to decide when to use a frame, and how to draw it.
  • If a flickable has both vertical and horizontal scrollbars, they overlap in the bottom-right corner. This looks wrong when the scrollbars have a background. A fix could be to resize the scrollbars differently when the background delegate is set/visible.
• GroupBox
  • Checkable groupboxes (a checkbox can be placed in the groupbox title)
• ToolButton
  • Missing a menu API. The button should be styled with an arrow if it opens a menu, so hacking this together from the app will probably not be enough?
• Slider
  • Missing tick-marks API
• Dial
  • Missing tick-marks API

Other things that needs to be done

• Make "default" style be the platform style. And rename "default style" to e.g "plain style" 
• Adjust all layout rects, and verify that controls look as expected with layouts
• Add cmake support
• Add native menus/popups (for combobox, tooltip, etc)
  • Read QPlatformTheme