workspacerules: Add a persistent workspace rule (#3530)

* add persistent flag to workspace rules

* get rid of unused ws->indestructible flag

src/helpers/Workspace.hpp

@@ -50,9 +50,6 @@ class CWorkspace {
     bool m_bDefaultFloating = false;
     bool m_bDefaultPseudo   = false;
 
-    // don't destroy in sanity check
-    bool m_bIndestructible = false;
-
     // last monitor (used on reconnect)
     std::string m_szLastMonitor = "";