channel selection dialog [issue 4]

This code adds a button to the plot context menu called "set channel list". This button opens a dialog to select the channels to plot from a tree structure. If there are channels already in the plot then they are populated in the dialog. The tree structure of the available channels is at the top left of the dialog. The list of channels to plot is at the top right. Channels can either be dragged and dropped into the list, or empty rows can be added with the insert button and edited. If a channel in the list does not exist then its text is turned red. Rows can be deleted from the list by selecting them and pressing the delete button. Rows can be rearranged by dragging and dropping them. The tree can be navigated by the keyboard arrows. Channels can be added to the list by pressing enter when they are selected.