Disabling Primary Selection per default

[Oliver Sauder]

Currently Primary selection support is activated per default.

This has mainly the advantage that also instant paste support works in apps which do not use Ctrl + V such as terminal.

However this also brings quite a few disadvantages amongst the following:
* flooding of clipboard history with unnecessary items
* higher power consumption (needs a constant timer to check what is selected)

Advantages and disadvantages indicate that primary selection support is an advanced feature. I herewith want to propose to disable primary selection support per default but can always be enabled in the preferences.