Replace another green loading gif with a grayscale one, by @mindeffects
Fix small glitch in nested tabs as seen in MIGX, bug introduced with the scrolling-fix for normal tabs
Fix JS error because of different mask objects in windows, reproduce by dropping a chunk in resource content
Fix excessive padding in quick update windows with tabs
Fix Element Tree windows / formfield widths
Fix scrollbars in the two different window types (plain / with tabs)
Make browser window height dynamic from viewport, make buttons consistent
Normalize windows, prevent oversized windows in smaller viewports, tested all windows again
Fix issue with duplicate method declarations in phpthumb.class.php
Get rid of weird character in phpthumb class
Fix grid mask overlapping topnav dropdowns, see #11711
Fix missing padding on TV panel > input properties, additional TV input props
Make sure fieldsets always have border-radius
Fix scrollbar issues in treedrop window
Correct icon classname for user image fallback

Without the "icon" class name, the fallback icon is not displayed properly.
Fix broken combobox pagination
Center pagination buttons regardless of listwidth
Add check for allow_url_fopen then CURL fallback

Add a check to see if allow_url_fopen is enabled then fallback to CURL if available.
Cleanup the resource getnodes processor

Prior to this cleanup, due to an issue with ltrim(), it was impossible for custom resources to use a custom icon. After this cleanup that ability is restored through a system setting (that 3PCs need to provide) of mgr_tree_icon_<CLASS_KEY> where <CLASS_KEY> is a lowercase representation of the class key with, if applicable, the "mod" prefix removed.

On top of that, this clean up also resolves duplicate classes being assigned to the icon. It also improves the readability and fixes some weird and/or duplicated logic.
Various manager theme improvements

Merge remote-tracking branch 'origin/pr/11699'

* origin/pr/11699:
  Make sure fieldsets always have border-radius
  Fix missing padding on TV panel > input properties, additional TV input props
  Fix grid mask overlapping topnav dropdowns, see #11711
  Fix small glitch in nested tabs as seen in MIGX, bug introduced with the scrolling-fix for normal tabs
  Replace another green loading gif with a grayscale one, by @mindeffects