Move main_actiongroup handling into MeldDoc and update children
filediff: Assume non-existant output files are writable (bgo#730194)

The scenario this handles is VCs that give a non-existant file as the
target file for a merge, expecting the helper (i.e., us) to write
that file if the merge was successful.
Update appdata file for license tag changes
build_helpers: Use distutils output helper
build_helpers: Update icon theme on install

This commit also introduces MeldDistribution as a semi-sane way of
passing the --no-update-icon-cache flag through to the install process.
build_helpers: Move broken gsettings schema flag to distribution
Updated Basque language
Fix loop on replace all

https://bugzilla.gnome.org/show_bug.cgi?id=729758
dirdiff: Fix "Same" filter for shallow comparison cases (bgo#730346)
preferences: Remove padding-only Alignment from editable list template
preferences: Improve layout of text filter preferences pane
preferences: Repack text filter preferences, and clarify trimming pref
WIP: Port to C++