Bug 41043: Hardcode the UI font on Linux

The mechanism to choose the UI font does not play well with our
fontconfig configuration. As a result, the final criterion to choose
the font for the UI was its version.

Since we hardcode Arimo as a default sans-serif on preferences, we use
it also for the UI. FontConfig will fall back to some other font for
scripts Arimo does not cover as expected (we tested with Japanese).

Bug 43141: Hardcode system-ui to Arimo.