mmg: make »scanned files« columns sortable

Fixes #954.
mmg: show sort indicators in »scanned files« list box

See #954.
refactoring: use InsertItem() with wxString as second argument
mmg: select scanned file dialog: right-align columns with sizes/durations
fix compilation with mingw wxWidgets
build system: don't store generated icon .h in the repo
build system: convert bin2h call from Ruby executable to built-in function
Merge branch 'sort-columns-in-playlist-selection-dialog'