Improve consistency of Configure/Settings page

- Change configure menu item text to "Configure Skanpage…" from "Settings" (like other KDE apps)
- Change default shortcut for configure to Ctrl+Shift+, from Ctrl+C (like other KDE apps)
- Change title to "Configure" from "Skanpage Settings" (like other KDE apps)
- Use Kirigami FormLayout's built-in labels instead of custom RowLayouts with Label and control
- Express min height and width in [grid units instead of pixels](https://develop.kde.org/hig/layout/units/)
- Make the "show all devices" checkbox a radio button
- Make folder selector look and behave like in other parts of KDE (eg Configure Dolphin > Startup)