Add "Scale" or "Zoom" option to Pdfcrop

[robamler]

• [x] Drop-down list with possibilities "70% (shrink)", "80% (shrink)", "90% (shrink)", "100% (normal)", "110% (enlarge)", "120% (enlarge)", and "130% (enlarge)". Effectively, this scales the -opdi by the given factor.
• [x] Store the setting in the journal database; not sure yet if new column can be added or if it has to be attached to k2pdfoptparams with a custom pseudo-option (prefer first solution if possible).
• [x] Report custom scale settings to server if user agrees
• [x] Add gui to edit scale settings in preference window.
• [x] Add scale settings to default journal database