Wip: Draw menu via QStyle

Right now we are just using a colored rectangle, which is ok with breeze but not with other styles.

• Overlap of submenus
• Color is wrong in Breeze because it uses a mix of Window and Base color but they are the same in palettes coming from Kirigami
• No gradient in Oxygen because it needs an actual widget for that (nothing we can do about that, if the style checks for a widget).
• Pixel artifact in Oxygen:
• Shadow (still need to hardcode)
• TODO: Separator
• Figure out a way to extend menuitems to the border of the menu
• Investigate missing space on the left of menu items