Add Selection tools and Colour Dropper, update theme

Selection tools can be used to select areas and then blank them out,
they will be used for copy and pasting once layer support is added.
ToolAttributes now have the functions isMutator() and isSelectable()
to support selection.

The Colour Dropper tool selects colours from the drawing. In order
to support this, the start(), move() and end() funcions of the
Command interface now return a ToolReport which contains details
of what the tool has done. It includes the Path traversed by the
tool so far in the current operation and also the colour of the
pixel touched in the current event.

Finally, the app has been rethemed. The icons have become white
and the palette menu item now has a white border to stay
consistent with the theme. The theme will continue to be
improved upon.
Merge branch 'feat/tool' into devel

Conflicts:
	app/src/main/java/com/jaween/pixelart/ContainerFragmentNarrow.java
Improve thumbnail scale, max/min zoom and gridline edge

The thumbnail is now scaled by the device's density.

The max zoom is now when there are at most 4 pixels along one
dimension of the screen, and min zoom is when the image becomes
width*dp by height*dp in dimension.

The right-most and bottom-most gridlines no longer flicker when
zooming.

Also fixed the zoom threshold to display the thumbnail.