some minor updates

Some minor improvements and bugfixes

DnD for filetree, and find & replace support for editor

bugfixes & improvements

some API changes & bug fixes

optimized image package; added some new widgets; new file explorer pa…

…ckage(file chooser & file tree); and other enhancements

add line number to editor, hightlight clicked line of editor

new TextField; new action bar

1. Optimize list implementation.

2. Optimize view dispatching and fix some routing issues.
3. Make modalView customizable;
4. Add option to let simpleNavItem be able to be shown as modal view.
5. Make tabview customizable.
6. Customizable menu background and option inset.
7. Update showcase.

1. Add treeview style file explorer

2. Add network image loading support.
3. Fix image loading and caching issues.
4. Fix context menu performance issues and keyboard events handling.
5. Other style/API updates.