[16.0][IMP] web_form_view_resizable: proporitonal min/maxWidth, resize with pdfpreview, jumping issue when resizing

[AnizR]

Some improvements over PR OCA#2976:

• Set min/maxWidth proportional to the size of the window
• When dragging to expand the form view over a pdf preview, it was a bit 'buggy': when hovering the div containing the preview while dragging, the scaling was stopping.
• When clicking on the handle to resize the form, there was a weird behaviour:
  "...I grab the resize handle it shoots to the right a couple of pixels first(like 100px or so)..." ([16.0][ADD] web_form_view_resizable OCA/web#2976 (review))