Add function atg_ebox_empty to detach & free all elements of a box
Port the filepicker widget to Windows

Uses scandir implementation based on one from Cygwin.  Not tested.
Fix bugs in atg_empty_box
Use atg_ebox_empty in atg_render_filepicker
Add atg_create_element_label_nocopy for mutable labels
Use nocopy labels in filepicker widget
Use nocopy labels in spinner widget
Update LVERSION to 2:0:1
Fix a bug in the filepicker
Add atg_create_element_button_image helper function