Fix bug: the on/off state of "Line backlight" item is not synced.
Merge pull request #19 from tyeen/toggle_enable_backlight

Fix bug: the on/off state of "Line backlight" item is not synced.
Remove ourself from the target of the shared NSColorPanel after adjusting color.
Merge pull request #21 from tyeen/shared_color_panel_issue

Remove ourself from the target of the shared NSColorPanel.
Add "Underneath Mode" menu item.

It should be excluded to "Overlay Mode"(the original "Enable Backlight"
option). And when its on, "Stroke" and "Radius" should not be effective.
Analogically, when both "Overlay Mode" and "Underneath Mode" are off, "Stroke"
and "Radius" options should be off, too.
Make the new "Underneath mode" work.

Basicly, "Underneath mode" uses NSLayoutManager's
addTemporaryAttribute:value:forCharacterRange: method to add a background
color to the line of the selectedRange.
Color of "Underneath mode" should be synced after changing.
Show the result of the adjustment in real time.
Merge branch 'underneath_mode' of github.com:tyeen/Backlight-for-XCode into underneath_mode

Conflicts:
	Backlight/AAABacklight.m