add a possibility to add and remove from selection in lasso and rect mode

[dieterdreist]

Currently I can add objects from a selection in rectangle and lasso mode by holding shift, or when holding alt JOSM will select also ways that are partially in the selection area. But I cannot get it to deselect.
Strange enough, "ctrl" makes the screen pan (this is typically done with "alt" in FreeBSD and Linux on the OS level, while many applications use "space" for it), ctrl is very strange for this.
Maybe it has to do with the broken modifier keys in MacOS (partially system conform, partially Java conform), I tried every key combination that came to mind but could not make it deselect.
What about adding a dedicated UI, something similar to how graphic programs work: you select the current selection mode with radio buttons between: replace, add, remove, (maybe toggle) and maybe also object type checkboxes (not radio buttons) for node, way, relation, and this would be the current selection mode if you do not hold any modifier keys (until you change the selection mode). Default is "replace" as it is now.