Don't deselect on Ctrl+Right-Click

(Part of my work towards !273)

Currently, when items are selected and a user right-clicks a selected item
while having the Ctrl key pressed down, the item is first
deselected and then a context menu is opened that doesn't involve
the item that was just deselected.

This is slightly confusing because normally one right-clicks an
item to see its context menu. Right-click being able to deselect
an item seems like unintended functionality in the first place but
in this scenario it also means that the intended opening of a
context menu for the pressed item doesn't even happen.

There is a good chance that nobody is even aware of this behaviour
because the normal way to deselect an item would be to
Ctrl+Left-Click if anything. Why would someone choose to open a
context menu and deselect items in a single step? Why would they
have selected an item they don't want to open a context menu for in
the first place?

Because of a discussion in this merge request deselecting on Ctrl+Middle-Click was also
removed for consistency with Ctrl+Right-Click and
Ctrl+Shift+Middle-Click.

This commit also makes a later implementation of a selection mode
more straightforward.