ToolBarSceneSelection should check playmode change

[Flavelius]

ToolBarSceneSelection's Draw handling of the dropdown conflicts with opening scenes for playing.
As this changes the lists content (rebuild), the scene is opened implicitly (again) by this action (GUI.changed).
This becomes noticable if the ToolbarStartFromFirstScene button is used. The internal reopening of the scene prolongs the playmode start and the regular unity toolbar buttons don't show the entering-playmode-state change (play-button being visually pressed down).
This little change works around this (in DrawSceneDropdown() - ToolbarSceneSelection.cs):
if ((scenePath) == scenesPopupDisplay[selectedSceneIndex].path && !EditorApplication.isPlayingOrWillChangePlaymode) // check for entering playmode here