widgets/notebook: the whole notebook need an update on set_current_by_index

Thank prife for the patch.

Reported-and-Tested-by: prife
widgets/notebook: fix a MDK compiling error
RTGUI version 0.6
widgets/window: fix a compiler warning in rtgui_win_show

Reported-by: huangxi_hans on the forum
app: add ref_count check in rtgui_app_exit

If the ref_count reaches 0, don't decrease it.
demo: fix demo_view_listctrl compiling error in MDK

Reported-by: znfc2 on forum
translate the README to English
check the fd for hz_file

It will print a warning when failed to open the font file.

Reported-by: prife, 文哥 and wnnwoo
add clip window test case
topwin: misc code formating
simplify the window showing algorithm

Children windows will always be shown when the parent window is shown.
test_cases/simple_win: misc cleanup
widget: fix clipping in onhide

Children should give back their clip, not extent. This fix
test_cases/clip_win.

Reported-by: xiao苦 in the forum
remove unused RTM_EXPORT

Reported-by: prife
Asked-by: qiuyiuestc
Github-hook: fix #31
topwin: re-draw the whole tree if a window in tree has been activated

When a window in tree has been activated, the whole tree will be raised
to the front. So we need to re-draw the whole tree in case some window
is covering other windows in the tree.
topwin: update doc
widgets: don't rtgui_widget_update_clip(win)

The clip of window is managed by topwin. Doing
rtgui_widget_update_clip(win) will mess it up. So add a sanity check on
it.

If windows are not widgets, things will get more clear.

Reported-by: xiao苦 in the forum
topwin: fix window modaling

This fix a bug that forgot to modal the chilren of root window.
add calibration to RTGUI

This moves the calibration.c from BSPs into RTGUI. The only thing that
the touch driver should do is to #include <rtgui/calibration.h> to get
the struct calibration_data definition.

It provide calibration_set_restore/calibration_set_after to give ways to
the user to save and restore the calibration data.
RTGUI version 0.6.1