Fix underline position and remove Inbox code

[sbrudz]

This PR focuses on fixing the issues with the position of the underlines for various edge cases, including:

• long emails with multiple line breaks
• small windows with the editable area scrolled horizontally or vertically
• different positioning in forward vs. reply

As part of this work, I unified how underline position calculation is done (so compose, reply, and forward all work the same way) and removed the Inbox specific code. The tests all pass now.

There are still two outstanding issues related to underline positioning:

• If the window is resized while the underlines are shown, the underlines will be out of place until the text is changed again
• Adding a line break between paragraphs does not trigger the underline position to be recalculated, so this can cause the underlines to be mispositioned until a character is typed into the edit box.

These issues are related to a different area of the code, so I'd like to handle them separately.

@ProfJanetDavis @steveatdm @perfetti Please let me know what you think of the changes. If you'd like any tweaks made to the line position (such as moving it closer to the word), please let me know.

Fixes #54
Fixes #55

[ProfJanetDavis]

Works for me! Thanks, @sbrudz !

[steveatdm]

@sbrudz Looks good, glad we could unify that. Only suggestion is to bring the underline a bit closer to the target text as you mentioned (Opened #61 which would change it from the left representation to the right):

[sbrudz]

That looks nicer, @steveatdm . Thanks for the PR!