Implement directory monitoring using septag/dmon

Use a notification based directory monitoring based on the
septag/dmon lirbary instead of periodically rescan the whole
project's tree.

resources/notes-dmon-integration.md

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+
+`core.set_project_dir`:
+    Reset project directories and set its directory.
+    It chdir into the directory, empty the `core.project_directories` and add
+    the given directory.
+    `core.add_project_directory`:
+        Add a new top-level directory to the project.
+        Also called from modules and commands outside core.init.
+        local function `scan_project_folder`:
+            Scan all files for a given top-level project directory.
+            Can emit a warning about file limit.
+            Called only from within core.init module.
+
+`core.scan_project_subdir`: (before was named `core.scan_project_folder`)
+    scan a single folder, without recursion. Used when too many files.
+
+Local function `scan_project_folder`:
+    Populate the project folder top directory. Done only once when the directory
+    is added to the project.
+
+`core.add_project_directory`:
+    Add a new top-level folder to the project.
+
+`core.set_project_dir`:
+    Set the initial project directory.
+
+`core.dir_rescan_add_job`:
+    Add a job to rescan after an elapsed time a project's subdirectory to fix for any
+    changes.
+
+Local function `rescan_project_subdir`:
+    Rescan a project's subdirectory, compare to the current version and patch the list if
+    a difference is found.
+
+
+`core.project_scan_thread`:
+    Should disappear now that we use dmon.
+
+
+`core.project_scan_topdir`:
+    New function to scan a top level project folder.
+
+
+`config.project_scan_rate`:
+`core.project_scan_thread_id`:
+`core.reschedule_project_scan`:
+`core.project_files_limit`:
+    A eliminer.
+
+`core.get_project_files`:
+    To be fixed. Use `find_project_files_co` for a single directory
+
+In TreeView remove usage of self.last to detect new scan that changed the files list.
+