Virtual selection fixes

This patch improves behaviour when selecting content in internet explorer. As an added bonus, the number of timeouts has been reduced, so the window's selection stays more closely in-sync with the cursor's selected range.

Additionally, it addresses a crash when a selection ends in a collapsed whitespace element.
Reproduction steps:
1) Create an ODT document with two paragraphs, the first ending with a whitespace (e.g., "p1 \np2")
2) Drag a selection starting at the end of p2, up to the end of p1, ending just before the '1' character
3) Observe the crash. This is caused by the window's selection being set to a non-visible whitespace by the shadow cursor

Fixes for another crash:
1) Using the previously created document, in webodf insert a space after the "1" character (e.g., "p1<s/> \np2")
2) Drag a selection starting at the end of p2, up to the end of p1, ending just before the '1' character
3) Observe the crash. This by the selection overlay attempting to use the BCR for a non-visible whitespace element