Split single command entry box into separate program and arguments.
The desktop entry "Exec" states: "A command line consists of an executable program optionally followed by one or more arguments." By having a single entry "command" encourages users to write shell commands like "FOO=123 someApp" rather than a legal "/bin/env FOO=123 someApp". This patch adopts the pattern used in kdevelop/hotspot/tools where we explictly ask for executable and arguments. CCBUG: 455252