diffgrid: Significantly rework size allocation

The previous method worked as long as there was exactly 2 rows, and there
were no widgets that spanned multiple columns. Unfortunately, that's
exactly what the FindBar does, in row 3.

The new model somewhat simplifies the grid allocation, and makes it more
generic by allocating by child using grid location data, rather than
doing allocation per-row, per-column (which doesn't work as soon as
widgets span multiple rows or columns).

There is still a lot of hard-coding of dimensions here, but this works
for now.
diffgrid: Refactor some variables
diffgrid: Fix pane width calculations and accumulate columns sensibly
Updated German translation
sourceview: Remove Alt+Left/Right keybindings, since we replace them
Update NEWS
conf: Post-release version bump
meldwindow: Save and restore window maximisation state (bgo#731693)
Updated Czech translation
Updated Polish translation
meldwindow: Manually handle app menu for non-Gnome shells (bgo#725430)

Since we don't have a global application menu bar (due to limitations
of GMenuModel, and having to totally redesign all of our menus and
rewrite associated code because of GtkUIManager deprecation), this
commit adds a fallback that manually recreates the application menu
in a UIManager-compatible form, and inserts it into the appropriate
(new) placeholder.
Updated Brazilian Portuguese translation
Updated Polish translation
WIP: Port to C++