[TIMOB-8110] MobileWeb: Create a new height/width constant of Ti.UI.INHERIT

There are a number of times where controls need to do either UI.SIZE or UI.FILL based on the parent's width/height. Right now this is accomplished manually, resulting in a lot of repeated code. We should implement a new constant of UI.INHERIT to avoid this code repetition. Ti.UI.INHERIT tells a control that it's width/height should be set to the parent's width/height, respectively. This operation is considered a raw copy of the user assigned value, not the calculated value. If the parent width/height is undefined, then the child inherits the parent's default width/height, which is always defined. If a parent's width/height or default width/height is also INHERIT, then the child goes up the element chain until it finds a width/height that is not INHERIT.