app: make error-console highlighting criterion configurable
Currently, the error console is highlighted (shown/blinked) only upon errors; however, warnings, which are not shown on the statusbar while the error console is open, often also contain important information. Allow the user to configure which message types (errors, warnings, and regular messages) highlight the error console, using a new "highlight" submenu in the error-console menu. Add corresponding config options, saved in sessionrc. By default, highlight the error console unpon both errors and warnings.
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