set colors in preferences dialog

[cdepillabout]

PR #130 (for issue #82) implemented a preferences dialog. It would be convenient to have a way to set console colors from the preferences dialog.

The preferences dialog currently looks like this:

It would be nice to add another tab to the preferences dialog so that it could set colors.

For reference, here is the color-setting dialog from Roxterm:

Although I think this is somewhat confusing. I would prefer it to look more like the output of the print-console-colors command, but each color would be editable:

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In order to implement this, you'd need to do the following:

1. Create something like a notebook with multiple tabs for the preferences dialog:

   termonad/glade/preferences.glade

   Line 13 in 7e4a62d

   <child internal-child="vbox">

   You should probably use Glade for doing this. It is pretty easy to use even if you've never used it.

2. Make the current preferences dialog the first tab.

3. Add a new tab for setting the colors. I'm not sure what the best layout for this would be, but something that allows the user to easily set all 256 would be ideal. Creating this layout programatically would probably be easier than doing it with Glade. Here's the function that shows the preferences dialog. This is what you'd need to edit if you're doing it programatically:

   termonad/src/Termonad/App.hs

   Line 714 in 7e4a62d

   showPreferencesDialog :: TMState -> IO ()