Little tweaks after months of feedback and use

Bugfixes:

• Corrected bug where the visible text area would not appear correctly
• Changed signal "finished_dialog" to "finished" to make it follow the animation player and other nodes signal naming pattern
• Some other less important bugs are now ironed out;

Enhancements:

• It is now possible to send a single text from the runtime to be shown, without the need to have it previously stored on the language file. For that, you just need to call show_text(chapter,dialog) with "single_text" as the chapter parameter, and write the string you want to show on the dialog parameter