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
• 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)

Screenshots:

How it looks now:

Dolphin folder selector, for reference: