Support HiDPI on Linux

[Kekun]

ISSUE
Steam doesn't follow the HiDPI mode set in GSettings, making it look super small and being barely usable.

OS
Fedora 25

STEPS

• Set HiDPI mode (for example run `gsettings set org.gnome.desktop.interface scaling-factor 2`)
  
• Start Steam

RESULTS
Steam looks super small compared to the other applications and is barely usable on an actual HiDPI screen.

EXPECTED
Steam's UI should be scaled accordingly to the GNOME scaling factor, at least when ran on GNOME.

[malachid]

With the Feb 23rd beta (don't know the version number because this bug prevents the window from being enough to see the entire thing) the main menu (Store | Library | etc) look a lot better. The menu bar above it is still very tiny, including the window operations on the top-right corner.

The main form content of this bug report takes up 3.5" width on my 17" monitor, leaving about 10" width of plain wasted background color. Each row of the normal text is about 1cm tall.

I am using the Dell M3800 preloaded with Ubuntu.