[shellprocess] Implement timeout setting

- For both shellprocess and contextualprocess, add a top-level key
   "timeout" that defaults to 10 seconds (which it already did).
 - Allows setting "global" timeout for command-lists, while still
   allowing individual timeouts per-command.
 - Setting timeout per global variable in contextualprocess is not
   supported; that would restrict the possible space of comparisions,
   while not supporting a global setting timeout seems reasonable enough.
   Use instances if you need wildly variable timeouts and don't want to
   set them individually.

src/modules/shellprocess/ShellProcessJob.h

@@ -46,7 +46,6 @@ public:
 
 private:
     CalamaresUtils::CommandList* m_commands;
-    bool m_dontChroot;
 };
 
 CALAMARES_PLUGIN_FACTORY_DECLARATION( ShellProcessJobFactory )