Store Street View sidepanel's width after resizing

[Glodenox]

With the recent addition to resize the sidepanel that contains the Street View imagery it would make sense to also store this position over sessions with a local storage variable. Right now it always defaults to just 50% of the width.

Things to account for:

• window resizing (store data as a percentage value instead?)
• listener might disappear when the WME enters another mode and the sidepanel gets regenerated
• either don't store the data on each trigger or make sure storing the data is very lightweight as there might be a lot of triggers while dragging the panel
• make sure that the street view pane doesn't need some sort of trigger so it knows it has been resized (check the Waze or Google code for this)

[Glodenox]

Initial implementation added in version 0.8.0. To test further later on.