Removed unused member variable

Detected by clang++ (-Wunused-private-field).
Fixed updating of Terrains list when adding/removing tilesets

The TerrainModel was not correctly notifying views about these changes.
Fixed crash when having multiple tilesets with terrain

The terrain brush will now refuse to look for ways to transition between
terrains from different tilesets, which is better than crashing in this
situation.
Removed explicit Rename Layer action

You can now rename layers using F2 only when the layer list has focus.
The advantage is that you can now also use F2 to rename objects or
terrains.
Show/Hide Other Layers action now uses undo commands

Needed for consistency with using undo/redo for toggling individual
layer visibility.
Fixed crash in terrain brush when dealing with unset tiles

This crash was introduced in 43fef0db20f3691714b88785c54211d47b23e7b0,
when the null check was removed from Tile::terrain.

The edge functions are only used by the terrain brush, so they have now
been moved there, with the null check reintroduced.
Replaced 'unsigned int' with 'unsigned'

I prefer brevity to verbosity in this case, they mean the same.
Replaced 'uint' with 'unsigned'

The 'uint' is apparently an old compatibility name, so let's not use it.
Added shortcuts for undo/redo in Edit Terrain Dialog
Tried to make erasing terrain information less confusing

Now the tileset view shows a gray overlay indicating where terrain
information is specified, and the "No Terrain" button has been replaced
with one saying "Erase", which does not affect the tileset view when
toggled.
Added 'E' as shortcut to toggle erasing terrain information
Fixed another potential terrain brush crash

After change 43fef0db20f3691714b88785c54211d47b23e7b0 there were some
more cases where a null check was missing.
Only highlight the terrain when it's not "no terrain"
Added an action to create a terrain given its display tile

Convenient shortcut to first adding a new terrain type and then setting
its display tile.
Updated NEWS file
Small terrain brush code tweaks
Removed unused New Image Layer dialog
Have lupdate remove obsolete entries
Updated all translation files

To give an idea of the size of this update, the lupdate output for most
translations was:

Found 517 source text(s) (130 new and 387 already existing)
Removed newimagelayerdialog.ui from the .pro file as well...