Do not grab click events on the eventTrap.

Fixes an issue where clicking on the middle of
the character to the right of the caret would
cause the cursor to move to the start or end
of the document.
Merge pull request #432 from adityab/eventtrap-clickungrab

Do not grab click events on the eventTrap.
Add test for frame position.
Add a helper function to get the bounding rectangle for an ODF page.
Improve positioning of frame that relative to page.
Add type that is a union of ClientRect and a struct with the same members.
This makes is possible to use them interchangeably.
In older browsers, positioning is only allowed on whole pixels.
The positioning test should set the allowed error higher in such browsers.
Add more supported length units.
And add a bit of documentation.
Check for null return value for getBoundingClientRect.
Slight clarification of exception.