UI: Fix issue #21: Resizing of vogleditor window is slow depending on trace size
* If the window is only being resized a small amount (within 20%), the timeline will no longer redraw and simply gets stretched.
* When the timeline is redrawn, it will now only draw one timeline item per pixel instead of many sub-pixel items.
* Fixed an issue that caused 0-duration items from being scaled to 1 instead of the horizontal scale value (which should be the minimum amount of time needed to appear as a single pixel). This greatly reduced the number of items being drawn.
* Fixed deletion of timeline item children.

* These improvements reduced the number of drawn items in one particular trace file from ~5.9 million, down to ~1250 in the default window size. When I stretched the editor across two monitors (giving the timeline more pixels to draw into) this increased to ~4500 items - both very reasonable numbers.