[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/Tests.cpp

@@ -64,7 +64,9 @@ ShellProcessTests::testProcessListSampleConfig()
     CommandList cl(
         CalamaresUtils::yamlMapToVariant( doc ).toMap().value( "script" ) );
     QVERIFY( !cl.isEmpty() );
-    QCOMPARE( cl.count(), 2 );
+    QCOMPARE( cl.count(), 3 );
+    QCOMPARE( cl.at(0).timeout(), -1 );
+    QCOMPARE( cl.at(2).timeout(), 3600 );  // slowloris
 }
 
 void ShellProcessTests::testProcessListFromList()