Fix crashes on database save

* Add saving mutex to database class to prevent re-entrant saving
* Prevent saving multiple times to the same file if the database is not marked as modified
* Prevent locking the database while saving. This also prevents closing the application and database tab while saving.
* FileWatcher: only perform async checksum calculations when triggered by timer (prevents random GUI freezes)
* Re-attempt database lock when requested during save operation
* Prevent database tabs from closing before all databases are locked on quit

src/gui/DatabaseWidget.h

@@ -80,6 +80,7 @@ public:
 
     DatabaseWidget::Mode currentMode() const;
     bool isLocked() const;
+    bool isSaving() const;
     bool isSearchActive() const;
     bool isEntryEditActive() const;
     bool isGroupEditActive() const;