Edit pictures: width × height with aspect lock, in px, cm, mm or inches #725

From @khankhanmohsin93-rgb in #645 (comment):

add option in the new slice - "Edit to" after the Half Size to resize the Image according to the user defined size in both width and height in any unit (px, inch, cm, mm, etc), lock or maintain aspect ratio (optional)

Today

• Presets can already resize to a free width × height, stretched or with the aspect kept ([Feature]: Custom image resize (free width × height, optional aspect-ratio lock) #576), in pixels only.
• The Edit pictures window (the slice after Half size) offers only Original size, Percent, Long edge and Width.

Plan

1. Width × height in Edit pictures: the same free size as the preset editor, with Keep aspect ratio (fit inside) or stretched to exactly that size. It reuses the preset's custom size, so nothing new is stored.
2. Units: px, cm, mm and inches. A size in cm, mm or inches needs a resolution to become pixels, so choosing one of them shows a DPI box (300 by default), and the saved picture records that DPI. Upload forms often ask for a printed size, e.g. a 3.5 × 4.5 cm photo.

Related: #662 (a file size range in KB, also for upload forms).