Disable software update

[letsfindaway]

For packages built for a distribution the update mechanism is different than for packages installed from the openboard.org web page. The automatic and manual software update check should be disabled. This involves:

[App]
HideCheckForSoftwareUpdate=true        <- change
EnableAutomaticSoftwareUpdates=false   <- change
EnableSoftwareUpdates=true             <- remove (obsolete and unused)

Additionally, the check box in the preferences controller enabling the software update check will be removed.

I want to provide this as a patch for others building packages, but not as a PR to the project, as this will never be merged. Therefore I will probably create a PR in my fork, so that the patch can be retrieved from there.

[letsfindaway]

See #117. The patch can be downloaded from https://github.com/letsfindaway/OpenBoard/pull/117.patch.