Dual pane preview

This PR adds dual-pane preview, supporting the following modes:

• Full-width (default, same as what exists now)
• Half-width
• Half-height
• Windowed

Additionally, scrolling is synced between the editor and preview using scrolling percentage, similar to similar editors (most notably, iA Writer). This can be disabled via a setting.

To enable this, a few other improvements have been made along the way, including:

• Major performance improvements to window resize / markup. For instance, scrolling is practically instant now, even when pasting very large documents.
• Characters-per-line is now a setting, which defaults to 66 per this recommendation. It can be tweaked via dconf/terminal. We can also expose this if we want to.
• Search in full-width preview was broken before, now it's disabled. In the future we can use webkit's search APIs to support searching in this configuration.
• Work-around for the very old issue of unwanted scroll while resizing the window. It is not a proper fix by any means (see description here), but it temporarily addresses the issue while we figure out a better solution.

Closes #55 (closed) Closes #59 (closed) Closes #141 (closed)

A short demo: