Updated clickable areas (1 bugfix + clickable in slave window support)

This pull request fixes a bug related to clickable areas, and adds clickable areas in the slave window.

Bugfix: title bar clicks stopped working after the slave window gots an update. Example command:
```
(echo "^ca(1, echo click)Header^ca()"; while true; sleep 1; do echo test$((i++)); done) | ./dzen2 -l 12
```
(The reason this happens is because xorig value is changed when the slave window got a redraw, but xorig is also used to determine whether a click happened on a clickable area).
New feature: clickable areas in the slave window.

Example code
```
(echo "^ca(1, echo TitleClick)Header^ca()"; while true; sleep 1; do echo "^ca(1, echo SlaveClick)test$((i++))^ca()"; done) | ./dzen2 -l 12
```
For performance reasons, the clickable areas for the slave window and title window are separated. When one window gets an update, the clickable areas for that window only are re-calculated.

robm / dzen