Switched to using MindmapNode everywhere

MindmapNode is a wrapper around (DOM) Node, with some additional logic.
It hides the complexity from Node and supports lazy loading from the
(fully loaded) DOM tree.
Factored the whole document loading into Mindmap
Bugfix: keeping last document open when starting from Launcher

When DroidPlane was started from the Laucher, it always opened the help.
I have now fixed this, so that the last document is reused (if it is
still in memory). If the memory has been flushed, the help is opened.
Added help menu
Prioritized TODOs
Merged from feature-custom-mindmap-tree

Merge commit '2c34f20c91bdfa0c3b8971d70ff52604a76770f8'

Conflicts:
	src/ch/benediktkoeppel/code/droidplane/HorizontalMindmapView.java
	src/ch/benediktkoeppel/code/droidplane/MainActivity.java
Removed all TODOs from source and added them to my mindmap
Pulled from master to get bugfixes and code cleanup

Merge branch 'master' into feature-context-menu

Conflicts:
	res/values/strings.xml
	src/ch/benediktkoeppel/code/droidplane/HorizontalMindmapView.java
	src/ch/benediktkoeppel/code/droidplane/MainActivity.java
	src/ch/benediktkoeppel/code/droidplane/MindmapNode.java
	src/ch/benediktkoeppel/code/droidplane/NodeColumn.java
Code cleanup of context menu