Task: Correctly handle bulk removal of virtual tags/depends attributes

This edge case would happen if a user issued a following command

    $ task 1 mod depends:
or

    $ task 1 mod tags:

which would not perform as expected for tasks with non-empty depends /
tags attributes.

The problem under the hood is the fact that current synchronization
between 'tags" attribute and its constituent decomposed `tag_X`
attributes is performed in a way where the set of tags obtained from
`tag_X` attributes is taken as the source of truth.

If the legacy 'tags:' attribute was set to empty, the fixTags() method
would then restore its previous value from the `tag_X` attributes,
effectively leading to a no-op.

The same happens with dependencies. The fix here is to detect removal of
depends and tags attributes, and instead of setting the legacy
attributes to empty values, we iteratively remove all tags/dependencies.

Closes #2655.

src/Task.cpp

@@ -2276,6 +2276,19 @@ void Task::modify (modType type, bool text_required /* = false */)
             value == "''"   ||
             value == "\"\"")
         {
+          // Special case: Handle bulk removal of 'tags' and 'depends" virtual
+          // attributes
+          if (name == "depends")
+          {
+            for (auto dep: getDependencyUUIDs ())
+              removeDependency(dep);
+          }
+          else if (name == "tags")
+          {
+            for (auto tag: getTags ())
+              removeTag(tag);
+          }
+
           // ::composeF4 will skip if the value is blank, but the presence of
           // the attribute will prevent ::validate from applying defaults.
           if ((has (name) && get (name) != "") ||