Version 0.4 -- RTF export, more bugfixes

Version 0.4 is released! This version includes RTF and plaintext export, as well as some bugfixes and quality-of-life features. Read on below~

• RTF and plaintext export are both available from the standard CTRL+E shortcut--simply save your file with the appropriate extension, or select the file type in the file dialogue.
• Hopefully fixed the "link bug" where, upon selecting a chapter and clicking "accept," a link would then connect to the wrong section.
• Boxes for editing section text will be greyed out and cannot be edited if a section is not selected.