Folder navigation UI (#689)

* Rough first pass at folder navigation UI for Reference Panel

* More UI

* Button icon size

* Tooltips

* Close popup after choosing a directory

* Tweak UI element positioning

* Fixes for background and reference image differences

* Only display supported formats

* Refresh page when reloading models

* Minor cleanup

* Disable buttons instead of hiding them

* 3-Sliced button mesh so we don't have to stretch

* The base implementation was causing an NRE

* Hide descriptions when buttons are disabled

* Prefab for 3-sliced button

* Refactor popup to use PagingPopUpWindow. Implement 3-sliced buttons

* Move "no files" panel so it doesn't obstruct folder nav. Change wording.

* Revert wording change. We'll only show this for truly empty folders

* Bug fixes and refactor

* Treat directory changes the same as page changes for model tab

* Refactor that breaks stuff (WIP)

* Fixes (seems to work at least for monoscopic)

* Minor refactor

* Correctly de-init xr subsystems

* Update Rider plugin

* I think the model preview logic is working properly now.

* ItemCount should be for the current directory

* Info text, UI tweaks

* Add missing popup to list on mobile prefab variant

* Fix Advanced Tools panel initial position

* Change how images are stored and looked up in save files (use relative path)

* A bit more consistency with other media formats

* Fix for model loading from subdirectories

* dotnet format