0.17.3 - more bugfixes and minor improvements

I've been working nonstop since last update to polish the tool further.  It's time for a break to do other things,
so here's a handful of small - but some are important - updates. 

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0.17.3
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New features:
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New NEXXT instance:

• Opens another instance of NEXXT. 
• Can be found in the File menu. 
• Hotkey: { Shift+Ctrl+N }.
• Tip: Working in 2 instances simultaneously unlocks a lot of NEXXT:s utility.
  You can for example use one canvas as clipbook for reusable structures,
  which you then can copy to clonestamp (v+left click) in your main instance.

Improvements:
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Save screen (32x30):

• Instead of being disabled when working on a map, the action now saves the currently viewed screen portion (32x30) of the map.

More events are undoable / revertable:

• Canvas size is now a part of the undo- and checkpoint buffers.
  As a result, it's now possible to undo (ctrl+z) or revert (shift+z) this action: Set canvas size { Ctrl+Alt+N }.
• In addition, you may revert (shift+z) any other "open file" action; if you set a checkpoint (shift+ctrl+z) before.

Bug fixes:
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Open CHR from File:

• Now links to the correct open dialogue (it didn't while open binary actions are organized in the file menu).

Save screen (32x30):

• In 0.17.2, "save screen" allowed to save any size by mistake.

Change canvas size - rare garbage tiles:

• If you changed a canvas so that it would be taller, but at the same time narrower,
  a remainder "garbage tiles" may show up on the first new extra row.
  Garbage tiles are now cleaned up before ending the action. 

Open by file association bug 1:

• Would crash while attempting to draw the Navigator canvas. Now fixed.

Undo/Redo bug:

• Undo/Redo properly swaps metasprite labels now.

Set/Revert checkpoint:

• Set/Revert now swaps metasprite labels (they didn't). 

Known bugs:
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Open by file association bug 2:

• After closing NEXXT with a file opened through file association, windows seems to retain the handle.
  Practically, windows won't allow deleting NEXXT.exe because "a file is open in the software",
  until reboot. Nothing obvious to me in the source seems to reflect this; all files seem to close after being read.
  I may of course be wrong. Needs to be studied.