NSIS: Only remove sections that we actually installed

We can query the NSIS Memento extension for that.
NSIS: Don't install link to Desktop of a link by the same name already exists

Unfortunately, we cannot determine the target path for the .lnk file
without pulling in closed source plugins (or write our own), a length
I am not willing to go in the light of MSI.

Fixes #356
NSIS: Cleanup: Use MEMENTO defines where possible
Merge pull request #1357 from owncloud/nsis_fixes

NSIS preparations for upgrade mode