Changes after upgrading to Slint 1.6

[Vadoola]

• Switch minimize window button over to minimize api added in 1.5 instead of using private slint backend Switch to new minimize and maximize api when they are released in stable #64
• Update ThemeBar component now that Slint has added individual corner radii for rectangles ThemeBar Component Layout #14
• Focusable items now have a clear-focus() function added in Slint 1.6, this might clean up my GHK focusing issues, see if I can use it to make the GHK components work better
• Look into the new getter for the raw-window-handle to see if i can use that and get rid of my use of the private slint backend. - Edit, I can't get rid of it, I still need it for moving the window and focusing the window when restoring from the system tray.