Include only existent files in RecentFiles list
Add some TODO items
Replace modules/editors.cpp with widgets::Workspace class

Now we have one editor for each Document we want to edit. Before we were
creating one editor and switching the document on it, but now we have
several Editors (one for each opened Document). This simplifies the
Document implementation, because it doesn't need to know temporal
Editor information (like "preferred editor settings").

- Removed all code from modules/editors.cpp (legacy from C code).
- Removed PreferredEditorSettings class and
  Document::getPreferredEditorSettings().
- Added Workspace/WorkspaceView classes.
- Added DocumentView (a DocumentObserver).
- Added MiniEditorWindow class.
- Removed SelectFileCommand & CloseEditorCommand.
- Added TabView interface instead of using a raw void* in
  Tabs/TabsDelegate classes.
- Modified editors_draw_sprite() calls to Document::notifySpritePixelsModified
  notifications.
- The "current_editor" global variable wasn't removed because it's
  used in several places yet, but it will be removed in the near future.
  (Also this variable now can be NULL when we don't have an opened
  document.)
Minor change in document_tab_popup labels
Add "play" button in MiniEditorWindow (UI only)

At this moment the button isn't functional because the current frame
position is stored in the sprite (instead of the Editor). In this case,
when the "play" button is pressed, the mini-editor should show other
frames instead of the current one (should play the whole animation).
Update copyright year to 2013
Fix bug in tools with TracePolicyLast: the old_dirty_area was accumulated between different ToolLoopManager loops
Fix bug where palette isn't correctly set when the DocumentView is changed
Refactor: move current frame/layer from Sprite class to Editor/DocumentView

This change is the first step to create a mini editor with the capability
to show the animation while we edit other frames. Also it will allow us
to have two or more editors of the same document in different frames.

- Added DocumentLocation: now the location in a document is a property
  of the current Context (Context::getActiveLocation()).
- Added ContextAccess/Reader/Writer classes.
- Added DocumentApi class: high-level API with the majority of methods
  in UndoTransaction class (that were not directly related to a undo
  transaction).
- Added AddFrame/RemoveFrame undoers to handle add/remove frame correctly:
  this fix issue #212.
- Removed SetCurrentLayer/Frame undoers.
MiniEditorWindow already handle the playback button correctly
Update Aseprite name and copyright year in README file
Now mini editor's frame is synced with current editor's frame
Add support to split workspace views

+ Added WorkspacePart class
+ Added WorkspaceView::cloneWorkspaceView()/onClonedFrom() methods.
Add Widget::getFirstChild() member function
Remove unused member Workspace::m_mainPart
Use Widget::getFirstChild() in Workspace::destroyPart()
Implement Workspace::makeUnique()