Feature request: add magnifying glass popup menu

[oeway]

Currently the magnifying glass works by right and left clicking, however, there is no right click with ImageJ.JS on mobile or tablet. One solution is to add an additional popup menu for selecting zoom in and zoom out explicitly.

[imagejan]

The magnifier tool implementation is in imagej1 in Toolbar.java:

https://github.com/imagej/imagej1/blob/c70a30b718126f5d8cd7b3fc754aa5395c9412cd/ij/gui/Toolbar.java

@rasband will have to decide how to adapt the implementation.

[rasband]

The ImageJ 1.53f43 daily build adds a magnifying glass tool popup menu and it supports long presses to trigger popup menus.

[oeway]

Thanks @rasband !