CustomParser: Add warning parser and output channel filter

There have been several requests on the mailing list or the
bug tracker to support parsing warnings for alien compilers
(sometimes slightly modified GCC).

Instead of natively supporting every compiler, users of less
frequently used compilers should use the custom parser to
parse errors and warnings.

The output channel filter for error and warning parser allows
to scan standard output, standard error or both channels.

Also added tests for twisted capture positions.

Task-number: QTCREATORBUG-11003
Change-Id: I5a5bd6f88cf21cde1c74962225067d4543693678
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

doc/src/projects/creator-projects-compilers.qdoc

@@ -150,6 +150,10 @@
 
             \image qtcreator-custom-parser.png
 
+            The custom error parser enables you to capture errors and warnings separately.
+            You can configure the error parser in the \uicontrol Error tab and the warning
+            parser in the \uicontrol Warning tab:
+
             \list 1
 
                 \li In the \uicontrol {Error message capture pattern} field, specify
@@ -163,6 +167,10 @@
                     expression groups to \uicontrol {File name}, \uicontrol {Line number},
                     and \uicontrol Message.
 
+                \li In the \uicontrol {Capture Output Channels} field, specify whether
+                    messages from standard output, standard error, or both channels
+                    should be captured.
+
                 \li In the \uicontrol {Test} group, you can test how the message that
                     you enter in the \uicontrol {Error message} field is matched when
                     using the current settings.