Split single command entry box into separate program and arguments.

David Edmundson requested to merge work/d_ed/split_program into master

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

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