Fix for mouse cursor overrides while resizing panels

[jbalsas]

Fixes a problem that overrides the cursor style when going over several elements while resizing a panel using the Resizer module.

Creates an auxiliar div during the resize operation on top of all the elements.

[redmunds]

Here's how to reproduce issue in master:

1. With sidebar showing, do Find in Files so bottom panel is showing
2. Start resizing height of bottom panel with mouse, and verify mouse cursor switches to row-resize cursor
3. While mouse is still down, move to the left over the edge of sidebar

Results
Cursor switches to col-resize cursor

Expected
Cursor should stay row-resize as long as we're in this operation

[redmunds]

Nice fix! Looks good. Merging.