Remove obsolete foldr resources/icons.
Updates svg icons in folder resources.

Fix compatibility issues with old xpm icons.
Add or update .xpm and .ico files for shortcuts and taskbars
Remove artwork folder.

* Replace use of wxICON by new icons declaration.
* Remove unused files.
* Fix filenames inconsistencies between old and new icons.
* dialog_erc handle variable size icons in erc matrix.

Fix minor other issues.
Remove unused files.

* Added checks for Inkscape and pngcrush when MAINTAIN_PNG mode is enabled.
* Rebuild PNGs with transparent background.