Adapt header buttons on low resolution displays

The header buttons are the major offenders when dealing with low
resolution displays, and they render the window unresizable even
on views that can resize.

In order to detect that, this commit adds a new method to check
if the current display has a low resolution. We're considering
anything below 800x600 as low resolution right now. This assumption
can change as we see fit.

The header buttons are adapted on realize, when it is guaranteed
that the window is in a monitor, and the fix is making the views
buttons non-homogeneous.

https://bugzilla.gnome.org/show_bug.cgi?id=787366