enhance the user experience with video sequencer

The idea is ,to set the end frame of the project to the end frame of the video that is added to the timeline , as it is by default set to 250 every time user has to set the end frame before working with that video and also when there is a video in the timeline and if the user want to import new video or movie it should be asking the user whether to place the video next to the video or movie that is already in the timeline or it has to placed where the video pointer is placed or at the beginning of the timeline. This improves the user experience to be beginner friendly and enhance the user experience to work in the video sequencer. Also show error message if no file is chosen for rendering video sequence.