Fix cast exception and revert regression.
Fixes icons with no mask (which is optional for 32BPP BMP icons) by checking if we are past the read boundary and then creating a dummy mask that is completely opaque.
Add undo/redo support to ActionList, does not yet revert edit values.
Implement move undo/redo
Merge pull request #164 from sorlok/fix_icons_no_mask

Fixes icons with no mask (which is optional for 32BPP BMP icons)
Cleanup and provide more efficient agorithms for some helper functions.
ActionList should start with no undo's.
Couple of fixes
* Moved the APNG reader from the plugin to LGM, you can now use it to load
all subframes of an APNG into a Sprite, pretty awesome.
* Fixed various flaws in some load dialogs when bogus filenames are
entered.
Add action list line numbers.
Update translations to be more coherent.
Missed one
Add save buttons to the background and sprite frames.
Fix translation issue.