Fluid: restructuring and rejuvenation of the source code.

* Add classes for application and project
* Removed all globals from Fluid.h
* Extracting args and project history into their own classes
* Moving globals into Application class
* Initialize values inside headers for some classes.
* Undo functionality wrapped in a class inside Project.
* File reader and writer are now linked to a project.
* Avoid global project access
* Nodes (former Types) will be managed by a new Tree class.
* Removed static members (hidden globals) form Node/Fl_Type.
* Adding Tree iterator.
* Use nullptr instead of 0, NULL, or 0L
* Renamed Fl_..._Type to ..._Node, FL_OVERRIDE -> override
* Renaming ..._type to ...::prototype
* Splitting Widget Panel into multiple files.
* Moved callback code into widget panel file.
* Cleaning up Fluid_Image -> Image_asset
* Moving Fd_Snap_Action into new namespace fld::app::Snap_Action etc.
* Moved mergeback into proj folder.
* `enum ID` is now `enum class Type`.

test/preferences.fl

@@ -138,8 +138,7 @@ Function {} {open return_type int
   code {readLanguagePrefs();
 Fl_Input::cut_menu_text = gettext("Cut");
 Fl_Input::copy_menu_text = gettext("Copy");
-Fl_Input::paste_menu_text = gettext("Paste");} {selected
-  }
+Fl_Input::paste_menu_text = gettext("Paste");} {}
   Fl_Window myWindow {
     label {My Preferences}
     callback closeWindowCB open
@@ -188,7 +187,7 @@ Fl_Input::paste_menu_text = gettext("Paste");} {selected
           xywh {0 0 100 20}
         }
         MenuItem {} {
-          label {flip flops}
+          label {flip flops} selected
           xywh {0 0 100 20}
         }
         MenuItem {} {