[gnome-software] (18 commits) ...Merge branch 'sidebar' into 'master'

[Philip Withnall <pwithnall src gnome org>]

Summary of changes:

  af88b0b... gs-shell: Remove unused variable
  8bf5bb1... gs-sidebar: Add new sidebar widget
  d1ca7e0... gs-shell: Rework shell UI, header bars and panes
  5fabb75... gs-shell: Rename header to main_header
  b128473... gs-shell: Drop buttonbox and page buttons
  fa24392... gs-shell: Show the search button on the installed apps page
  5973c7e... gs-shell: Drop ‘previous’ button
  df22584... gs-shell: Rearrange headerbars
  ab8585c... gs-shell: Drop unused widget
  70bc354... gs-shell: Add adaptive support to the headerbar
  3311915... gs-shell: Add separator between main content and sidebar
  7c3c1de... gs-shell: Connect category switching to the sidebar
  0b989b1... gs-shell: Tidy up visibility of back button
  9617baa... shell: Bind updates counter to needs-attention property of 
  e019a70... gs-shell: Drop Unity workarounds
  fd25d5e... style: Drop unused needs-attention class
  46c8aa4... gs-shell: Hide the back button by default
  fa502b9... Merge branch 'sidebar' into 'master'