Selection support using middle mouse button

[stoecker]

Currently it is very hard so select a specified object when objects overlap.

Suggestion is to use middle mouse button to cycle through the objects:

• middle mouse click 1 opens the dialog as it does now
• second click selects the first element in the list
• next click selects the next element in list an deselects previous
• one click more than elements in list deselects last element
• next click again selects first element
• When SHIFT is pressed already selected elements should be skipped (do ADD mode).
• When CTRL is pressed selecting an already selected element deselects it (TOGGLE mode).
• When moving the mouse the actions should be final (CTRL+SHIFT modes)

Probably the move threshold for dialog closing most be adapted. It seems to be a little low, so mouse handling must be very exact at the moment.