Revert "use a gloabl size group for track-header ctrl buttons"

This reverts commit 86eb72955c76575b75a2b9e535162ca7e0612bfd.
pass-though PixFader tweaks API for scroll events
fix track-header vertical scrolling

Ignore vertical scroll events on horizontal
Automation Track Sliders. see also 70be12afaa6
fix ArdourButton initialization
fix mixer strip track number display (amend 4a00bdf0)
gtk style related fixes:

* remove some old/unused styles
* fix plugin-ui button (hover color when active)
* consistent style for route buttons
  (and related ArdourButton updates)
optimize ArdourButton
speed up session load (display updates)
NOOP, cleanup whitespace
idle update editor TreeView

dramatic speedup for solo & route toggle.
fix another blinking-rec btn issue (amend 0ae8dd4)