Factor out some pixbuf loading

* Removes duplicate code
Add some doc comments [NFC]
freehand: correct log message, deprioritize

* Make the log chatter debug-only
* Correct a typo in the message for GTK beyond 3.12
Remove UI for layer group isolation

* Remove the user-facing toggle action (will be replaced with a mode)
Layers stack: add Pass-through mode

Re-express the absence of group isolation as a mode in its own right, to
simplify the user interface. This replaces the toggle for group isolation,
which was making the UI messy and hard to grok.

* New mode: "Pass-through", for layer groups only
  - named after the one in a certain large photo manipulation package
  - should be fairly self explanatory
  - hides lots of pointless internal complexity, yay!
* Remove isolated flag and group auto-isolation test (much rejoicing)
* Move UI strings to lib.layer
* Pass-through mode prevents the opacity being set, so grey out UI bits
* Update doctests
* SetLayerMode command now also saves & restores previous opacity
* Remove the update() mechanism for SetLayerMode
  - the update is no longer atomic, so it's trickier to code now
  - update() was dubious anyway: users may want to compare back'n'forth
* Mark a few internal constants with underscore prefixes