Switching to relative imports to allow pyqtgraph to be imported under other names.

finished top-level files and graphicsItems
updated widgets
console, graphicsscene, and 2nd-level graphicsitems
exporters, multiprocess, opengl
flowchart, canvas
dockarea, imageview, metaarray
.ui files
rebuild UI files
fixed circular import
Merge branch 'relative_imports' into develop

This merge converts all absolute imports into relative imports throughout pyqtgraph.
This allows pyqtgraph to be embedded as a sub-package of larger packages.
Added documentation on using pyqtgraph as a subpackage.
Fixes against relative-import reorganization
minor setup corrections
Major updates to ComboBox:
- Essentially a graphical interface to dict; all items have text and value
- Assigns previously-selected text after list is cleared and repopulated
- Get, set current value
PlotNode control widget now displays combo box to let user select plots to connect to
Flowchart example updated to use this feature.
fixes for python3
Merge branch 'flowchart_plotnode_menu' into develop