Add TaskList plugin

This plugin enables loading of .tasks files into the Build Isssue list. This
is really nice if you need to e.g. browse results from static codecheckers
in creator.

Tasks files are simple tab-separated lists of one of the following forms:
description
type\tdesciption
file\ttype\description
file\tline\ttype\tdescription

The strings may contain escape codes:
\n for newline,
\t for tab
and \\ for a backslash

src/plugins/projectexplorer/showineditortaskhandler.cpp

@@ -46,6 +46,8 @@ ShowInEditorTaskHandler::ShowInEditorTaskHandler() :
 
 bool ShowInEditorTaskHandler::canHandle(const ProjectExplorer::Task &task)
 {
+    if (task.file.isEmpty())
+        return false;
     QFileInfo fi(task.file);
     return fi.exists() && fi.isFile() && fi.isReadable();
 }