qutebrowser: add proxy-pac to qt.args

This defines which proxy (if at all) to use for which urls. It requires
qutebrowser to be patched to not load it's own proxy handling as that
seems to overwrite this.

This used to be set with QTWEBENGINE_CHROMIUM_FLAGS environment variable
in an untracked qutebrowser wrapper in PATH. It caused some issues with
darkmode though, as it seems that this env variable is not overwritten
when already set, and the correct darkmode settings are not being passed
to blink.

Setting the proxy file as qt.args works also with darkmode.