[FR] "Add movable projection plane"

[Pietersielie]

Is your feature request related to a problem? Please describe.
A number of features common in other cube visualisation software remain either missing or frustrating to use in iDaVIE. For example, visualising a single channel can be done by cropping to that channel, but visualising the channels in sequence is tedious and extremely time-consuming. Looking at a position-velocity slice is impossible.

Describe the solution you'd like
A movable projection plane in the cube, highlighted by wireframe perhaps, with the projection then displayed in a separate window (like moment maps are already). This plane should be both movable and rotatable, with perhaps a selection of algorithms (specified in config file) used to calculate the non-trivial sampling (e.g., karma's kpvslice algorithm).

Additional context
This feature could also be used to overlay one dataset on another, e.g., an optical image on top of the radio cube. The general idea has been mentioned in previous issues #74 and #197.