Only hide the browser selection once edit-mode is enabled

The recent shadow cursor implementation hides the native browser selection
highlights. However, the shadow cursor itself only starts being drawn
after edit-mode is enabled, meaning for a readonly document the selection
is invisible (#119)

This is a simple workaround that allows the native selection to be drawn
in circumstances where the shadow cursor is not active. Long term, the
shadow cursor should be also used on non-editable documents as detailed
in solution ideas for #119.
Clean-up the selection view when removing it

When a cursor is removed from the document (e.g., exiting edit mode),
the most recent selection is left visible.

* Don't re-append the overlayBottom on destroy
Upgrade from magical empty-string to const for shadow cursor memberId

In various parts of the code, the magical empty-string cursor id is used
to determine whether the object is a shadow cursor or not. Converting
this into a constant instead and removing the constructor arg prevents
some mistakes being made with this.
Merge pull request #122 from peitschie/native-selection

Only hide the browser selection once edit-mode is enabled
Separate annotation handling logic into a tool class AnnotationManager and UI into a editor/widgets/annotation.js

Also enable "Add annotation" button only if not inside annotations
Add keyboard shortcut to create an annotation
Merge pull request #130 from kossebau/limitAnnotationButton

Refactor annotation button code, also disabling it inside annotations (+ keyboard shortcut)
Move attribute "styleid" into new namespace "urn:webodf:names:helper", no longer leak into saved files
Use new namespace "urn:webodf:names:helper" also for attribute "containsparagraphanchor"
Merge pull request #121 from kossebau/fixStyleId

No longer write helper attribute "styleid" when saving
Clear any image selection chrome when exiting edit mode

Additionally, since the browser selection is no longer rendered, there
is no longer any reason to NOT synchronise it with the cursor selection.
Merge pull request #132 from peitschie/image-select-clear

Clear any image selection chrome when exiting edit mode
Provide access to more direct text styling functions

External editing features require some direct access along side the
eventing mechanism currently exposed. Additionally, there are more formatting
options available than DirectTextStyler supports in native functions.

By allowing access to the underlying function used to perform cursor
styling, external editors can easily provide support for custom text style
features (e.g., background colours etc.).
Merge pull request #131 from peitschie/direct-styling-tweaks

Provide access to more direct text styling functions
ISSUES.txt no longer used
Make the text:line-break elements behave like <br>