Use native file browsers on GTK-based linux environments

[thomashampson]

Description of work
Install an extra pacakge from conda-forge called qt-gtk-platformtheme. This package provides the GTK+ 3 platform theme, which makes Qt use native icon themes, fonts, and dialogs on GTK-based environments.

Purpose of work
Since moving to Conda, the file browsers on IDAaaS have become unusable when navigating to large directories. This is because the native browsers weren't being used. Installing this extra package solves the issue.

Further detail of work
Had to change the occt pinning in the conda recipes in order to resolve the dependency chain. It is now consistent with what was already in the developer environments (which is what is used to run all of our tests!).

To test:
(On linux only) Create a new conda environment, activate it, and install mantidworkbench:

conda create -n mantid-gtk-test
conca activate mantid-gtk-test
mamba install -c "thomashampson/label/gtk-test" mantidworkbench

Then launch with workbench and open up a file browser and navigate to a big directory. E.g. this one was causing problems on IDAaaS:
/archive/NDXLARMOR/Instrument/data/cycle_22_3/
The GUI should now remain responsive while the file browser is populating.

I have tested myself on IDAaaS by creating a new conda env and it is working well.

Optionally you can update your development conda environment after checking out this branch with
mamba env update -f mantid-developer-linux.yaml --prune
which will install the new package. Then when you launch your developer build of workbench you should see the gtk theme file browser.

Fixes #34820

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[robertapplin]

Works well for me, good stuff! Just wondered if it is worth adding a comment next to the qt-gtk-platformtheme package in the linux yml file?

[robertapplin]

Looks good to go