Add Switch File button to ppcomp dialog

[windymilla]

1. Saves the messages currently visible in the ppcomp dialog. Does not save messages that have been hidden.
2. Switches which file is loaded: HTML or text
3. Re-creates the dialog
4. Populates the dialog with the saved messages, but swapping display of text and HTML line numbers
5. Re-selects the same message that was selected before

Fixes #1725

[tjeffress]

Ideally we wouldn't need to close and re-open the ppcomp window each time we switch files, but since it's saving the content and re-opening, it achieves the goal of having the ability to switch files and still have the content available. I tried several different file sets and didn't experience any issues.

I especially like that the highlight colors were added to the color palette in the settings because I've always used ppcomp with the text file first and HTML second, so I'm used to the HTML being green and the text being purple. But since I can switch the colors, I don't have to retrain my brain to the color meanings.