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
Dummy context menu works

- finding out which node was pressed is not easy. Currently implemented
  very very ugly
- don't know which of the actual items in the context menu was selected
Started to extend LinearLayout in MindmapNode

the idea is that I can use MindmapNode directly as element in the
NodeColumn adapter. However, at the moment, it is not really possible to
load the mindmap_node_list_item.xml (by inflating it), because then the
node-specific details are not set, and when creating a node (without
inflating), the layout is not there.

I will have to find out how to get a MindmapNode, which inflates its
layout from the XML layout file, and still has all the stuff set that we
need.
Handling clicks in context menu works

- copy node text
- open link (if node has a link)
MindmapNode is now a proper GUI element

- new icon (plus/minus when expandable/contractable)
- MindmapNode can be added as GUI element. This happens in the
  NodeColumn ListView at the moment
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