Enforce ids for panel, content, move handle and resize handle

[safaorhan]

Background

In the current state, user gives ids of panel, content, move handle and resize handle using XML attributes:

panel_view, panel_content, panel_move_handle and panel_resize_handle.

However in the default layouts for left/top/right/bottom snaps the panel and content ids are hardcoded.

This creates a serious bug: When default snap layouts are used with ids different than panel and content snap layouts do not work and the panel cannot be snapped.

TODO

• Expose predefined ids for panel, content, move and resize handles and enforce those ids.
• Remove panel_view, panel_content, panel_move_handle and panel_resize_handle attributes.
• Update API docs if necessary.

Notes:

• This will make a breaking change.