popular tile: Use GtkStack for switching between states

Instead of manually hiding and showing widgets to switch between the
loading indicator and the actual app tile, use a GtkStack.

This avoids the resizing of the tile when the stars widget gets shown.
app tile: Use GtkStack for switching between states
feature tile: Use GtkStack for switching between states
popular tile: Avoid resizing tile icons during loading

Reserve 64x64 px for the icon image to avoid resizing the tile during
loading.

https://bugzilla.gnome.org/show_bug.cgi?id=734785
feature tile: Split out static CSS into gtk-style.css

Instead of applying all CSS at runtime, split out a part of the CSS so
that we can apply it at the app startup, avoiding unwanted resizing due
to CSS changes during loading.

https://bugzilla.gnome.org/show_bug.cgi?id=734785
Disable GtkButton's internal padding for tile buttons

... and add the same using margin properties instead. This paves way for
the next commit, giving us to better control where to place items in the
tile buttons.
popular tile: Adjust the "Installed" overlay label

... as per designer guidance.

https://bugzilla.gnome.org/show_bug.cgi?id=734781
Release version 3.13.91
trivial: Post release version bump
Updated Polish translation