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).