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
Layers dialog: update icons

* Groups now show a folder icon
* Add a blank icon for painting layers, for stupid layout reasons
versioning: set up for next release cycle

* Release script can now make pre-release tarballs too
* Tarballs have git revision info baked into them
* Builds with no .git use baked-in release info if present
* Document new versioning & release strategy
  - now use Semantic Versioning
  - prerelease tarball verions will compare correctly
  - version always suffixed by (potential) prerelease during development
  - only the final release commit is unsuffixed
* Slightly meaningless version bump to 1.1.1-alpha
  - this really means the same thing as "git after 1.1.0", same as now
  - its betas & release will probably be 1.2.0, given the amount of change
  - it has to be semantically after 1.1.0 :)
mypaint head merge