wmllint GUI: used a smaller icon for the Browse button, and made the entry fill the whole frame
wmllint GUI: made two labelframe labels shorter
wmllint GUI: removed verbs from option labelframe labels
wmllint GUI: moved the exit button to the right side of the window
wmllint GUI: verbosity level radiobutton, Normal -> Terse
wmllint GUI: avoid showing a blank Tk window if the program is run outside of a Wesnoth directory