Remove the ability to manually move required execution environments in
the Manifest Editor. Instead always sort the values in ascending Java
version order.

From a technical perspective the order of the values of the
'Bundle-RequiredExecutionEnvironment' header is irrelevant, similar to
for example the 'Import-Package' header.
Removing the buttons to reorder the values simplifies the UI.

Values of existing Plug-in projects are displayed in sorted order
regardless of if they are actually sorted in the MANIFEST.MF or not. On
the next write all values will be written in sorted order.