clarify creating and editing a plant

[niccokunzmann]

@Heineken identified the following behavior to bring clarity to when a plant is created. For a discussion, see #299.

• When I edit an existing plant, there is a button named "Delete". When I press it, the plant is gone.
• When I open a new plant from the map, there is a button named "Cancel". When I press it, the plant is gone.
• When I open a new plant from the list of my plants, there is a button named "Cancel". When I press it, the plant is gone.
• When I edit an existing plant and I press the back button, no changes are applied to the data or, if changes have been applied immediately while editing, they are reverted.
• When I open a new plant from the map, when I press the back button, the plant is gone.
• When I open a new plant from the list of my plants, when I press the back button, the plant is gone.
• When changes are discarded, there is a dialog informing about this and one can cancel discarding the changes.
• When a plant is deleted, a dialog asks if the plant should really be deleted.

Source code:

• Mainly: NewPlantActivity.java
• to switch between "create" and "edit", one can either subclass or use if/else or use a strategy pattern

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