[UX] Flexible templates UI: Allow "visual" region width changing, using left/right drag handles, and width indicators

This has actually been on my mind since the introduction of the flexible layout templates, but never gotten to file an issue for it till now.

Currently, one has to click the "Configure row" button, then click the "Change region widths" link, and then select from the available combinations from the dialog:

How about something like this instead?:

The handle would not increment by steps of 1% or anything like that, rather than "snap in place" while being dragged, allowing the user to select between the same predefined widths as before ...just in a way that is more direct/visual, and where the user can quickly configure region widths without having to open/close dialogs, and click though multiple links/buttons to achieve the same result.