Implement basic list sorting

- Can now sort VirtualListView lists by left-clicking column headers. Sorting defaults to ascending, clicking the same column again will set it to descending, and again will disable sorting.
- Currently there is no indication of what column is currently sorted, and there doesn't seem to be a nice way to do it with wxwidgets either, will need to look at this later.
- Haven't implemented custom sorting for texture editor textures and patches lists, so sorting by anything but name doesn't work too well.