qbs project files: Add project property ide_source_tree.

The qmake project has this as well. Will become more useful later on.

Change-Id: I6e9b6b1d1a46ea6263a6d5fac4cb30d883a3e077
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

qtcreator.qbs

@@ -9,6 +9,7 @@ Project {
     property string ide_compat_version_minor: '8'
     property string ide_compat_version_release: '81'
     property string qtcreator_compat_version: ide_compat_version_major + '.' + ide_compat_version_minor + '.' + ide_compat_version_release
+    property path ide_source_tree: path
     property string ide_app_path: qbs.targetOS.contains("osx") ? "" : "bin"
     property string ide_app_target: qbs.targetOS.contains("osx") ? "Qt Creator" : "qtcreator"
     property string ide_library_path: {

share/share.qbs

@@ -29,7 +29,7 @@ Product {
         name: "Conditional"
         qbs.install: true
         qbs.installDir: project.ide_data_path + "/externaltools"
-        prefix: "../src/share/qtcreator/externaltools/"
+        prefix: project.ide_source_tree + "/src/share/qtcreator/externaltools/"
         files: {
             var list = [
                 "lrelease.xml",