AutomatableModel: reset value to default if data is unavailable in loadSettings()

When loading older projects where a particular model did not exist and
thus did not save any data do not load the possibly invalid default data
but reset to default value of model.
Revert "Revert "Extensions for peak controller. Fulfills request 144.""

Now that we solved the problem with wrong default values in newly
introduced models when loading older projects we can safely implement
the new PeakController features.

This reverts commit 9cb52ccf0d84b31945b86cbf419db854c0d91204.
AutomatableModel: renamed to controllerConnection() + coding style fixes

Renamed getControllerConnection() to controllerConnection().
MMP: write to temporary file first and then rename

In order to avoid data loss because of full disks or other storage
failures write a temporary file first, rename current file to backup
file and rename temporary file to current file.

Closes #26.