UI refresh

This is a huge branch. A probably-incomplete list of major changes includes:

• Remove all GtkUIManager code, replacing it with GActions everywhere
• Remove menubar-based UI in favour of current GTK+-style headerbar + hamburger menu
• Move filters to popover menus, support status filtering in popovers, and add file filters
• Move as much common UI as possible to the header bar
• Remove all now-unused status and version control action icons
• Rework the find bar UI to better match current styling
• Removal of the filechooser-based UI for file comparison in the per-file toolbar

Many of the changes here depend on each other for a coherent UI. For example, the rewrite of the file comparison chunk action UI to not be part of the textview gutter was needed so that we could move to using overlay scrollbars for textviews, which also allowed us to move the chunk map over to the right hand side.

This UI refresh is not complete. The UI for displaying paths in the file toolbar is still in flux, and a similar UI change needs to happen for both folder and version control comparisons. The version control action toolbar needs some UI reconsideration. However, at this point, it's clear to me that the refresh has passed the point where there's any chance that it's not going to end up being the next version of Meld, so... may as well merge it.