Incorrect behaviour of default output directory on first run

[chrisjbillington]

Upon first running runmanager after install, when there is no labscript file loaded from a saved config file, it incorrectly has the warning about a non-default output directory displayed:

Upon subsequently choosing a labscript file, it doesn't set the output directory correctly:

The "set default output directory" button is even greyed out. To workaround, one must choose an output directory manually, then the "set default output directory" button is enabled and can be used to restore things to sanity.

[philipstarkey]

So this is due to the fix for #69. I think possibly that fix went about it in the wrong way. I think #69 only happened when the labscript file was set when loading in a config (either at startup or manually) because selecting a new labscript file via the GUI called check_output_folder_update() which should always have updated self.previous_default_output_folder so the explicit addition of updating that variable in on_labscript_file_text_changed() is unnecessary.

Possibly the fix should be to remove check_output_folder_update() from on_select_labscript_file_clicked() and modify on_labscript_file_text_changed() to call check_output_folder_update().