extension: Honour the scale factor for HiDPI

This fixes the second part of #84: Vertical offset when using HighDPI
terminal: Remove leftovers for gnome-shell 3.10+

Gnome-Shell 3.10.0 features VTE 0.34.8 so also drop fixes and
workarounds for older versions.
extension: Use the panelBox ::allocation signal

The panel actor allocation signal is not always sent with the various
other extensions. They almost all play with the panelBox actor instead,
like in the hide-topbar extension.

Switch to that allocation signal instead and throttles all the geometry
changes but more quickly as it seems not such a big deal comparing to
the past.

This fixes #35: Space at the top when top panel hidden