Added path-tool icons.
Fixed WorldLookup not working with negative coordinates.

WorldLookup uses unsigned coordinates, but rects/polgons passed as search
areas may have negative coordinates.
NewWorldDialog lets user choose path/regular world plus OSM file.
Give every path scene a tile grid.

The tile grid now skips every N'th gridline when the scale is small.

Worked around buggy OpenGL widget's painting of grid lines, something to do
with scale and negative coordinates.

Fiddling with the scale of non-tile path scenes as well.
PathDocument adds all texture in the Textures directory.
Added TextureBrowseDialog.
Fixed z-order of TilePathScene tile layers.
Fixed WorldLookup script indices.
Removed obsolete menu commands from MainWindow related to path textures.
Fixed PathWorldReader crash when an error occurred.
Fixes to some path tools.
Made all path-texture modifications undo-able.
Misc commits.
Removed qApp->processEvents call in MapReader.

Maps are loaded in threads now.
Fix crash when a building has tile layers not in TMXConfig.txt.

The user might add tile layers to TMXConfig.txt, then paint some tiles in
those layers in BuildingEd.  Creating a BuildingMap from that building would
crash if TMXConfig.txt did not have those layers.

I need to display a warning when a .tbx files has tile layers not in TMXConfig.txt.
Only run BMP blending code on parts of the a being displayed.

The blending code used to run on an entire map when one was loaded, which
was noticibly slow.  Now the blending code is run only on the parts of the
map being displayed at any given time.
Fixed some g++ compiler complaints.
Fixed WorldLookup deleting subtrees when it shouldn't.
Fixed hang waiting for a map to load.

If a map was loaded asynchronously but processing was deferred, and the same
was was later loaded non-asynchronously, loadMap() would wait forever.
TextureEditDialog won't change values the user is editing.

Changing the value in a spinbox will update the path in realtime, which would
cause the dialog to update its controls.