Improved tutorial dialog

Implement prototype of improved welcome/tutorial dialog. Some ideas:

• QML based to make it more dynamic.
• Implement proper page indicator instead of it being part of images. This way adding new page doesn't require updating all, indicator can be made functional etc.
• Slice images into smaller parts. This way total consumed disk space should decrease, current 1x + 2x images require ~2.8Mb. Also would make it possible to animate page changes.
• Maybe "Do not show this again" checkbox could be visible (at bottom-left corner?) all the time, so user doesn't need to do selection at first page.