ProjectExplorer: Add "Re-detect" button to toolchains page

This allows to take changed detection settings or added/removed
compilers into account without restarting Qt Creator.
It is also the only way to get rid of formerly auto-detected compilers
that would not get detected anymore with the current settings.

Change-Id: I3d8a7659e24fa5d55d1fb17864386cf19d3e2533
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/toolchainsettingsaccessor.cpp

@@ -195,6 +195,8 @@ QList<ToolChain *> ToolChainSettingsAccessor::restoreToolChains(QWidget *parent)
     const QList<ToolChain *> systemFileTcs
             = toolChains(restoreSettings(FileName::fromString(Core::ICore::installerResourcePath() + TOOLCHAIN_FILENAME),
                                          parent));
+    for (ToolChain * const systemTc : systemFileTcs)
+        systemTc->setDetection(ToolChain::AutoDetectionFromSdk);
 
     // read all tool chains from user file.
     const QList<ToolChain *> userFileTcs = toolChains(restoreSettings(parent));