Fix combine_karaoke

[moi15moi]

This PR resolves: https://github.com/Ristellise/AegisubDC/issues/36

I have no idea what the person who created the combine_karaoke method was thinking. To me, the previous results make no sense, so I changed its behavior to match what I think it should do. I have listed many examples to show what I think the result should be.

Example 1 - The line has no collision and the timing of the second line is after the first line:

Dialogue: 0,0:21:18.86,0:21:21.00,ED,,0,0,0,,{\k50}Ko{\k47}no {\k51}ki{\k26}mo{\k40}chi
Dialogue: 0,0:21:22.78,0:21:25.26,ED,,0,0,0,,{\k34}ko{\k20}{\k49}i {\k24}de{\k54}su {\k67}ka

Before this PR (different k-time compared to the two lines):

Dialogue: 0,0:21:18.86,0:21:25.26,ED,,0,0,0,,{\k214}{\k50}Ko{\k47}no {\k51}ki{\k26}mo{\k40}chi{\k248}{\k34}ko{\k20}{\k49}i {\k24}de{\k54}su {\k67}ka

After this PR (same k-time)

Dialogue: 0,0:21:18.86,0:21:25.26,ED,,0,0,0,,{\k50}Ko{\k47}no {\k51}ki{\k26}mo{\k40}chi {\k178}{\k34}ko{\k20}{\k49}i {\k24}de{\k54}su {\k67}ka

Example 2 - Lines have timing collision:

Dialogue: 0,0:21:18.86,0:21:21.00,ED,,0,0,0,,{\k50}Ko{\k47}no {\k51}ki{\k26}mo{\k40}chi
Dialogue: 0,0:21:19.78,0:21:25.26,ED,,0,0,0,,{\k34}ko{\k20}{\k49}i {\k24}de{\k54}su {\k67}ka

Before this PR (different k-time compared to the two lines):

Dialogue: 0,0:21:18.86,0:21:25.26,ED,,0,0,0,,{\k50}Ko{\k47}no {\k51}ki{\k26}mo{\k40}chi {\k34}ko{\k20}{\k49}i {\k24}de{\k54}su {\k67}ka

After this PR (same k-time)

Dialogue: 0,0:21:18.86,0:21:25.26,ED,,0,0,0,,{\k50}Ko{\k47}no {\k51}ki{\k26}mo{\k40}chi {\k-122}{\k34}ko{\k20}{\k49}i {\k24}de{\k54}su {\k67}ka

Example 3 - The line has no collision and the timing of the first line is after the second line:

Dialogue: 0,0:21:40.86,0:21:50.86,ED,,0,0,0,,{\k50}Ko{\k47}no {\k51}ki{\k26}mo{\k40}chi
Dialogue: 0,0:21:19.78,0:21:25.26,ED,,0,0,0,,{\k34}ko{\k20}{\k49}i {\k24}de{\k54}su {\k67}ka

Before this PR (different k-time compared to the two lines):

Dialogue: 0,0:21:40.86,0:21:50.86,ED,,0,0,0,,{\k1000}{\k50}Ko{\k47}no {\k51}ki{\k26}mo{\k40}chi{\k548}{\k34}ko{\k20}{\k49}i {\k24}de{\k54}su {\k67}ka

After this PR (different k-time compared to the two lines):

Dialogue: 0,0:21:40.86,0:21:50.86,ED,,0,0,0,,{\k50}Ko{\k47}no {\k51}ki{\k26}mo{\k40}chi {\k-3108}{\k34}ko{\k20}{\k49}i {\k24}de{\k54}su {\k67}ka

But, please note that Join (concatenate) also doesn't work in this third case!