Restore margins of Application, URL and Permissions tabs in the Properties widget, and change checksums QLabel to QLineEdit for wrapping

Something must have broken before from Frameworks 5 to 6, and there are no longer proper margins in most tabs. This makes specifically URL and Application properties tabs look quite bad, as they end up with text glyphs right up against the edge of the window. I have removed the explicit 0px margins, instead leaving the layouts to inherit from the stylesheet. QLabel in KChecksumsPlugin is now QLineEdit, as this shows a nice looking box, and doesn't expand the window by an insane amount. It's set to readonly, so it isn't editable.

BUG: 484789

before images -

after images -