Adjust Ajax loading spinner position within dropbuttons

[laryn]

Description of the bug

From an issue in the Gin queue which seems to actually be a core issue: the Ajax spinner bloats the dropbutton height when active.

Steps To Reproduce

To reproduce the behavior:

1. Go to Layouts and create a flexible layout template. Give it a name and save it to load the page where you can adjust regions.
2. Click on Configure in the header region
3. See the dropbutton expand strangely while the popup loads

Actual behavior

Expected behavior

The spinner should load next to the dropbutton's text without expanding the height.

[indigoxela]

@laryn many thanks for starting this topic. (Oh, this ugly throbber... 😜)

Looking at your PR - it may be safe (it hopefully is), but may also be a breaker, as it applies to all AJAX requests via jQuery using a throbber.
This really needs thorough testing with all sorts of usages.