WIP: Add paste preview dialog

Reasoning:

1. Trick pages like this can infect you're computer with virus.

   ls -la<br>
   <span style="font-size: 0">echo "You got virus!"</span><br>
   ps -aux

2. Bash, Python, etc. multi-line editing is awful, it cannot be used.

KTextEditor problems:

1. Can't navigate out of the editor using keyboard only.
2. Ctrl+S opens the save dialog.
3. Kate font is used instead of the system monospace font.
4. Does not follow the system color scheme, light color scheme is always used.

TODO:

• Make dropEvent() use doPaste(), drop processTextBeforePasting()?
• Add a Context menu.
• Allow configuring tab width.