Support custom content view scale & offset when show menu

In iPad landscape case, the content view will be scaled to very small & the menu's width will be too wide. It's better to offer a way to modify the scale & offset values for the content view when show menu.

This PR adds `contentViewScaleValue`, `contentViewInLandscapeOffsetCenterX` & `contentViewInPortraitOffsetCenterX` to support defining custom scale &
offset of content view when show the menu.

The default value of them still use the original values:

- contentViewScaleValue = 0.7f;
- contentViewInLandscapeOffsetCenterX = CGRectGetHeight(self.view.frame) + 30.f;
- _contentViewInPortraitOffsetCenterX  = CGRectGetWidth(self.view.frame)  + 30.f;