Add configuration option for default build directory

Change the UI to always show the editor for the build directory.
Default to what we do for qmake projects right now (with shadow-
building enabled).

Use this setting in qmake based projects.

Set this to '.' for in-source builds by default.

Change-Id: I3fb26ab817b0f545e9b318b68e592df8040d5562
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/coreplugin/documentmanager.h

@@ -129,9 +129,6 @@ public:
     static QString projectsDirectory();
     static void setProjectsDirectory(const QString &);
 
-    static bool useBuildDirectory();
-    static void setUseBuildDirectory(bool use);
-
     static QString buildDirectory();
     static void setBuildDirectory(const QString &directory);