Update to work with VisPy 0.7 (#312)

* Update minimum required vispy version to 0.7

Includes fixes for removed vispy modules

* Update TiledGeolocatedImageVisual to work with VisPy 0.7

* Remove unused shader code for basic Image layer

* Update CompositeLayerVisual to work with VisPy 0.7

* Refactor custom ImageVisuals to prepare for VisPy-compatible MultiBandImageVisual

* Remove unnecessary _MultiBandTextureAtlas2D delegate/wrapper class

* Extract non-SIFT-specific MultiChannelImageVisual stuff to subclasses and mixins

This should allow us to move the MultiChannelImageVisual to vispy-core in the future

* Add vispy-compatible tests for MultiChannelImageVisual

* Fix unnecessary ArrayProxy usage

* Refactor fill array for MultiChannelImageVisual

* Fix flake8 issues

* Use QtWidgets instead of QtGui for some base classes and decorators

* Fix two additional QtGui->QtWidgets misattributions

Co-authored-by: R.K.Garcia <r.keoni@gmail.com>