[2.3] Manager Theme: UI Improvements Windows, Forms, TVs and more

Here it is, sorry for the delay @integrators, **I'll update this description with the usual screenshots and screencaps, submitted like this that you can start reviewing**.

The integrators are gonna hate me for this one (again) and I want to apologize in advance and promise to not do such huge PRs again!

**In short, this PR does the following:**

- Completely restyled/refactored and outsourced SCSS for windows to fit into the theme (new file _windows.scss)
- Replaced ExtJS/Javascript animations for opening windows with CSS3 transitions
- Refactored the styles for forms
- Fontawesome icon replacement for checkboxes and radios in forms (not in grids yet)
- Fontawesome icons for Comboboxes (normal, date, time, image, file, code) including fix for misaligned arrows from #11617
- Completely refactored the styles for the Multiselect TV
- Make URL TV fit into the UI
- Re-styled the tags for tag TVs
- Normalize TV widths
- Placed the tv-reset icon left to the TV label again (wasn't looking good for fields with more height)
- Added fileupload.js which provides a new field xtype of "fileuploadfield" for the import Windows (can be used in extras too)
- Enhanced the resource quick update window to provide the full functionality of the first two tabs of the resource edit screen
- Browsertesting and Fixes for IE8 where possible, a lot more needs to be done...
- As the uberbar contains a form (searchfield) too, I did some minor fixes to the uberbar
- Tested all custom TV types I could find (Image+, MIGX, Fileupload TV, Simple File Upload TV, GeoTV, Daterange TV, Timerange TV)
- Cleaned up colors_and_vars.scss and the dependent .scss files
- Added more missing primary-button classes in panels
- Fixed layout breaking when too many tabs are in one panel (ex. Access Control Lists > Edit user group), re-enabled and restyled the tab-scrolling functionality
- Adjusted Dashboard Widget Header Bg Color to the one from the reworked grids


Fixes: https://github.com/modxcms/revolution/issues/11634, https://github.com/modxcms/revolution/issues/11635, https://github.com/modxcms/revolution/issues/11640, https://github.com/modxcms/revolution/issues/11668, https://github.com/modxcms/revolution/issues/11662 (partially, many more left)