Hide association end information when association is not visible

[Matthias Schoettle]

When two classes are moved on top of each other, they are considered "offscreen" and the association is not shown. However, components that are not removed (like multiplicity and role name) but just moved to a new position, are still visible. See the following screenshot:

The view should just be invisible in that case to hide everything.

[Jerry Wei]

This issue was fixed in my fix for Issue #325, upon improving based on Cecile's suggestion (see diagram in Issue #325)

[Jerry Wei]

Resolves #299: Hides association end information when association is not visible

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