Resizing objects improvement

**Is your feature request related to a problem? Please describe.**
Resizing objects is very frustrating currently. In particular, finding the handles for very large objects (spanning multiple screens) on a hi-dpi monitor can take half a dozen seconds each time, heavily harming productivity when many resizes are needed. This is especially true for side handles, which can be completely off-screen and hard to locate (first attached screenshot).

**Describe the solution you'd like**
Add a way to smart-select the closest handle on a selected object by holding right-click.

**Describe alternatives you've considered**
* Rework the handles to be located inside the objects to cover the entire side of the objects, similar to the selection tool in GIMP (second attached screenshot).
* Add a setting to make the handles larger.

hard to see handle

hovering on a resize handle in GIMP

No source file or test is named. Reproduce object resizing in Tiled on a hi-DPI display, especially with objects spanning multiple screens, then trace the desktop editor's selection and resize interaction entry points. Done means a selected object can smart-select the closest resize handle while holding right-click, with the behavior covering hard-to-reach side handles.

Written by the indexing model from the issue text.