New FlexibleItemDecoration

[davideas]

The new item decorator should implement identical drawing technique of DividerItemDecorator (from Android API); Should have equal item offset; Section gap offset; Should work with all layouts and orientations.

For item spacing technique, the open source decorator ibosong/CommonItemDecoration can be merged.

Details of the functionalities:

• Supports all RecyclerView LayoutManagers that implement (Linear, Grid or StaggeredGrid).
• Can add equal space offset at all sides of an item.
• Can customize the offset for each view type.
• Can customize the offset at the 4 edges: first and last column / first and last row.
• Recognizes the sections of FlexibleAdapter and can add gap offset between them.
• Recognizes the orientation of the current Layout.
• Supports the default Android divider android.R.attr.listDivider.
• Supports a custom divider by DrawableRes id.
• Supports drawing the divider over or underneath the items.