Replace kResizeMessage message with ResizeEvent and Widget::onResize()

- Remove jwidget_set_rect() function.
Remove jwidget_get_rect() and jwidget_get_child_rect() functions

+ Added Widget::getClientChildrenBounds()
+ Replaced all member functions in Theme::draw_x() to Theme::paintX()
+ Move code to draw tabs from Tabs::onProcessMessage() to Tabs::onPaint().
Fix issue #237: fix pen size for square shape

+ Add Pen::getBounds().
+ Add options to increment/decrement angle with ChangePen command.
+ Add App:PenAngleBefore/AfterChange signals.